Positive Electrode Active Material, Method for Manufacturing Positive Electrode Active Material, and Secondary Battery

ABSTRACT

A positive electrode active material which can improve cycle characteristics of a secondary battery is provided. Two kinds of regions are provided in a superficial portion of a positive electrode active material such as lithium cobaltate which has a layered rock-salt crystal structure. The inner region is a non-stoichiometric compound containing a transition metal such as titanium, and the outer region is a compound of representative elements such as magnesium oxide. The two kinds of regions each have a rock-salt crystal structure. The inner layered rock-salt crystal structure and the two kinds of regions in the superficial portion are topotaxy; thus, a change of the crystal structure of the positive electrode active material generated by charging and discharging can be effectively suppressed. In addition, since the outer coating layer in contact with an electrolyte solution is the compound of representative elements which is chemically stable, the secondary battery having excellent cycle characteristics can be obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/885,350, filed May 28, 2020, now pending, which is a continuation ofU.S. application Ser. No. 15/638,449, filed Jun. 30, 2017, now U.S. Pat.No. 10,741,828, which claims the benefit of foreign priorityapplications filed in Japan as Serial No. 2016-133143 on Jul. 5, 2016,Serial No. 2016-133997 on Jul. 6, 2016, Serial No. 2017-002831 on Jan.11, 2017, Serial No. 2017-030693 on Feb. 22, 2017, Serial No.2017-084321 on Apr. 21, 2017, and Serial No. 2017-119272 on Jun. 19,2017, all of which are incorporated by reference.

TECHNICAL FIELD

One embodiment of the present invention relates to an object, a method,or a manufacturing method. The present invention relates to a process, amachine, manufacture, or a composition of matter. One embodiment of thepresent invention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, a lighting device, anelectronic device, or a manufacturing method thereof. In particular, oneembodiment of the present invention relates to a positive electrodeactive material that can be used in a secondary battery, a secondarybattery, and an electronic device including a secondary battery.

In this specification, the power storage device is a collective termdescribing elements and devices having a power storage function. Forexample, a storage battery such as a lithium-ion secondary battery (alsoreferred to as secondary battery), a lithium-ion capacitor, and anelectric double layer capacitor are included in the category of thepower storage device.

Note that electronic devices in this specification mean all devicesincluding power storage devices, and electro-optical devices includingpower storage devices, information terminal devices including powerstorage devices, and the like are all electronic devices.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithium-ionsecondary batteries, lithium-ion capacitors, and air batteries have beenactively developed. In particular, demand for lithium-ion secondarybatteries with high output and high energy density has rapidly grownwith the development of the semiconductor industry, for portableinformation terminals such as mobile phones, smartphones, tablets, andlaptop computers; portable music players; digital cameras; medicalequipment; next-generation clean energy vehicles such as hybrid electricvehicles (HEV), electric vehicles (EV), and plug-in hybrid electricvehicles (PHEV); and the like. The lithium-ion secondary batteries areessential as rechargeable energy supply sources for today's informationsociety.

The performance required for lithium-ion secondary batteries includesincreased energy density, improved cycle performance, safe operationunder a variety of environments, and longer-term reliability.

Thus, improvement of a positive electrode active material has beenstudied to increase the cycle performance and the capacity of thelithium ion secondary battery (Patent Document 1 and Patent Document 2).

REFERENCE Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2012-018914-   [Patent Document 2] Japanese Published Patent Application No.    2015-201432

DISCLOSURE OF INVENTION

Development of lithium ion secondary batteries and positive electrodeactive materials used therein is susceptible to improvement in terms ofcharge and discharge characteristics, cycle characteristics,reliability, safety, cost, and the like.

An object of one embodiment of the present invention is to provide apositive electrode active material which suppresses a reduction incapacity due to charge and discharge cycles when used in a lithium ionsecondary battery. Another object of one embodiment of the presentinvention is to provide a high-capacity secondary battery. Anotherobject of one embodiment of the present invention is to provide asecondary battery with excellent charge and discharge characteristics.Another object of one embodiment of the present invention is to providea highly safe or highly reliable secondary battery.

Another object of one embodiment of the present invention is to providea novel material, a novel active material particle, a novel secondarybattery, or a formation method thereof.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects can be derived fromthe description of the specification, the drawings, and the claims.

To achieve the above objects, in one embodiment of the presentinvention, two kinds of regions which are different from a region insidethe positive electrode active material are provided in a superficialportion of the positive electrode active material. It is preferable thatthe inner region contain a non-stoichiometric compound and the outerregion contain a stoichiometric compound.

In addition, it is preferable that the inner region contain titanium andthe outer region contain magnesium. Furthermore, these two kinds ofregions may overlap.

In addition, it is preferable that the inner region be formed through acoating process such as a sol-gel method and the outer region be formedby segregation due to heating.

One embodiment of the present invention is a positive electrode activematerial including a first region, a second region, and a third region.The first region is present inside the positive electrode activematerial. The second region and the third region are present in asuperficial portion of the positive electrode active material. The thirdregion is present in a region closer to a surface of the positiveelectrode active material than the second region is. The first regioncontains an oxide of lithium and a first transition metal and has alayered rock-salt crystal structure. The second region contains anon-stoichiometric compound containing an oxide of a second transitionmetal and the non-stoichiometric compound has a rock-salt crystalstructure. The third region contains a compound of representativeelements and the compound of representative elements has a rock-saltcrystal structure.

In the above structure, it is preferable that the first transition metalbe cobalt, the second transition metal be titanium, and the compound ofrepresentative elements be magnesium oxide.

In the above structure, the third region may contain fluorine.Furthermore, the second region and the third region may each containcobalt.

In the above structure, it is preferable that crystal orientations ofthe first region and the second region be partly aligned with each otherand crystal orientations of the second region and the third region bepartly aligned with each other.

In the above structure, a degree of a mismatch between a (1-1-4) planeof the layered rock-salt crystal structure in the first region or aplane orthogonal to the (1-1-4) plane and a {100} plane of the rock-saltcrystal structure in the second region is preferably less than or equalto 0.12, and a degree of a mismatch between the {100} plane of therock-salt crystal structure in the second region and a {100} plane ofthe rock-salt crystal structure in the third region is preferably lessthan or equal to 0.12.

Another embodiment of the present invention is a positive electrodeactive material containing lithium, titanium, cobalt, magnesium, oxygen,and fluorine. When the concentration of cobalt which is present in asuperficial portion of the positive electrode active material and ismeasured by X-ray photoelectron spectroscopy is 1, the concentration oftitanium is greater than or equal to 0.05 and less than or equal to 0.4,the concentration of magnesium is greater than or equal to 0.4 and lessthan or equal to 1.5, and the concentration of fluorine is greater thanor equal to 0.05 and less than or equal to 1.5.

Another embodiment of the present invention is a method for forming apositive electrode active material including: a step of mixing a sourceof lithium, a source of cobalt, a source of magnesium, and a source offluorine; a step of heating the mixture of the source of lithium, thesource of cobalt, the source of magnesium, and the source of fluorine at800° C. or higher and 1100° C. or lower for 2 hours or longer and 20hours or shorter to obtain particles containing lithium, cobalt,magnesium, oxygen, and fluorine; a step of dissolving titanium alkoxideinto alcohol; a step of mixing the particles containing lithium, cobalt,magnesium, oxygen, and fluorine into the alcohol solution of thetitanium alkoxide and stirring the mixed solution in an atmospherecontaining water vapor; a step of collecting precipitate from the mixedsolution; and a step of heating the collected precipitate at 500° C. orhigher and 1200° C. or lower in an atmosphere containing oxygen under acondition where a retention time is 50 hours or shorter.

In the above formation method, a ratio of the number of atoms of lithiumin the source of lithium to the number of atoms of cobalt in the sourceof cobalt is preferably greater than or equal to 1.00 and less than1.07.

In the above formation method, a ratio between the number of atoms ofmagnesium in the source of magnesium and the number of atoms of fluorinein the source of fluorine is preferably Mg:F=1:x (1.5≤x≤4).

In the above formation method, the number of atoms of magnesium in thesource of magnesium is preferably greater than or equal to 0.5 atomic %and less than or equal to 1.5 atomic % of the number of atoms of cobaltin the source of cobalt.

In the above formation method, lithium carbonate, cobalt oxide,magnesium oxide, and lithium fluoride can be used as a source oflithium, a source of cobalt, a source of magnesium, and a source offluorine, respectively.

When the surface of the positive electrode active material is coveredwith a coating film to protect the above crystal structure, a decreasein capacity due to charge and discharge cycles can be suppressed. As thecoating film covering the surface of the positive electrode activematerial, a coating film containing carbon (a film containing a graphenecompound) or a coating film containing lithium or a decompositionproduct of an electrolyte solution is used.

In particular, powder in which the surface of the positive electrodeactive material is coated with graphene oxide using a spray dryapparatus is preferably obtained. The spray dry apparatus is amanufacturing apparatus using a spray dry method by which a dispersionmedium is removed by supplying a hot wind to a suspension.

When charge and discharge cycles are repeated, deformation of theparticles of the positive electrode active materials, such as crackingor breaking, might occur. It is said that such deformation makes a newsurface of the positive electrode active material exposed, and thesurface is in contact with an electrolyte solution to cause adecomposition reaction or the like, so that the cycle characteristicsand the charge and discharge characteristics of the secondary batteryare degraded.

Thus, a coating film is preferably provided to prevent the deformationof the particles of the positive electrode active materials, such ascracking or breaking.

However, when suspension is formed and stirred by a rotary andrevolutionary mixer to coat the surface of the positive electrode activematerial whose weight per unit volume is large with graphene oxide whoseweight is relatively small, coating is insufficient.

Thus, to coat the surfaces of the particles of the positive electrodeactive materials with the graphene oxide, a method in which the grapheneoxide and a polar solvent (such as water) are mixed and ultrasonictreatment is performed, the particles of the positive electrode activematerials are mixed therein to prepare the suspension, and dried powderis produced with a spray dry apparatus is preferably used. The driedpowder produced in this manner is referred to as a composite in somecases.

The size of one drop of spray liquid sprayed from a nozzle of the spraydry apparatus depends on a nozzle diameter.

When the particle diameter is smaller than the nozzle diameter, thereare a plurality of particles in one drop of the spray liquid sprayedfrom the nozzle. When the surface of the particle after drying under thecondition where the largest particle size is smaller than the nozzlediameter is observed, there are some portions where the surface iscoated with the graphene oxide; however, the coating is insufficient.

The nozzle diameter of the spray dry apparatus is preferablysubstantially equal to the largest particle size of the active materialbecause the coverage of the active material can be improved. Moreover,the largest particle size of the positive electrode active material ispreferably adjusted to be substantially equal to the nozzle diameter informing the positive electrode active material.

Since the graphene oxide is well dispersed into water, the suspension ofwater and the graphene oxide can be formed by stirring using ultrasonicwaves. The positive electrode active material is added to thesuspension, and the suspension is sprayed with the spray dry apparatus,whereby powder in which the surface of the positive electrode activematerial is coated with the graphene oxide can be obtained.

Note that the suspension becomes more acidic as the amount of grapheneoxide is increased. Thus, part of the surface of the positive electrodeactive material (e.g., LiCoO₂ containing Mg and F) might be etched.Then, a hydrogen ion exponent (pH) of the suspension before beingsprayed is preferably adjusted to be close to approximately pH7, thatis, close to neutral, or higher than or equal to pH8, that is, alkaline.For the pH adjustment, a LiOH aqueous solution is preferably used. Forexample, in the case where LiCoO₂ is used for the positive electrodeactive material and only pure water is used as the dispersion medium ofthe suspension, the surface of the positive electrode active materialmay be damaged. Thus, a mixed solution of ethanol and water is used asthe dispersion medium of the suspension, whereby damage to the surfaceof the active material may be reduced.

The suspension is formed in the above manner, whereby the positiveelectrode active material whose surface is coated with the grapheneoxide can be prepared efficiently. When the surface is coated with thegraphene oxide, the deformation of the particles of the positiveelectrode active materials, such as cracking or breaking can beprevented. Moreover, even if the positive electrode active materialwhose surface is coated with the graphene oxide is exposed to the airafter the formation, the change of properties or degradation can besuppressed. Here, “after the formation” refers to a period from thetermination of the formation of the positive electrode active materialto the start of the fabrication of the secondary battery containing thepositive electrode active material and includes the storing, thetransporting, and the like of the positive electrode active material. Inaddition, when the coating film is formed, the positive electrode activematerial and the electrolyte solution can be prevented from being indirect contact with each other to react; thus, the secondary batteryusing the coating film has high reliability.

For the spray dry method, a known apparatus can be utilized, forexample, a countercurrent pressure-nozzle-type spray dry apparatus and acounter-cocurrent pressure-nozzle-type spray dry apparatus can beutilized.

Note that the graphene oxide coating the surface of the active materialmay be reduced when used in the secondary battery. The reduced grapheneoxide is referred to as “RGO” in some cases. In RGO, in some cases, partof oxygen atoms remains in a state of oxygen or atomic group containingoxygen that is bonded to carbon. For example, RGO includes a functionalgroup, e.g., an epoxy group, a carbonyl group such as a carboxyl group,or a hydroxyl group.

Another embodiment of the present invention is a secondary battery whichincludes a positive electrode containing the above-described positiveelectrode active material or the above-described positive electrodeactive material coated with a coating film and a negative electrode.

The secondary battery can have any of a variety of shapes to fit theform of the device to be used, for example, a cylindrical shape, arectangular shape, a coin-type shape, and a laminated (flat plate)shape.

According to one embodiment of the present invention, a positiveelectrode active material which suppresses a reduction in capacity dueto charge and discharge cycles when used in a lithium ion secondarybattery is provided. In addition, a secondary battery with excellentcharge and discharge characteristics is provided. In addition, a highlysafe or highly reliable secondary battery is provided. In addition, anovel material, a novel active material particle, a novel secondarybattery, or a formation method thereof is provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C show examples of a positive electrode active material;

FIGS. 2A and 2B illustrate crystal structures of a positive electrodeactive material;

FIG. 3 illustrates crystal structures of a positive electrode activematerial;

FIGS. 4A-1, 4A-2, 4A-3, 4B, 4C, 4D-1, and 4D-2 show a sol-gel method;

FIGS. 5A to 5C illustrate a segregation model of elements contained in apositive electrode active material;

FIGS. 6A to 6D illustrate a segregation model of elements contained in apositive electrode active material;

FIGS. 7A and 7B are cross-sectional views of an active material layercontaining a graphene compound as a conductive additive;

FIGS. 8A to 8C illustrate a method for charging a secondary battery;

FIGS. 9A to 9D illustrate a method for charging a secondary battery;

FIG. 10 illustrates a method for discharging a secondary battery;

FIGS. 11A to 11C illustrate a coin-type secondary battery;

FIGS. 12A to 12D illustrate a cylindrical secondary battery;

FIGS. 13A and 13B illustrate an example of a secondary battery;

FIGS. 14A-1, 14A-2, 14B-1, and 14B-2 illustrate examples of secondarybatteries;

FIGS. 15A and 15B illustrate examples of secondary batteries;

FIG. 16 illustrates an example of a secondary battery;

FIGS. 17A to 17C illustrate a laminated secondary battery;

FIGS. 18A and 18B illustrate a laminated secondary battery;

FIG. 19 is an external view of a secondary battery;

FIG. 20 is an external view of a secondary battery;

FIGS. 21A to 21C illustrate a formation method of a secondary battery;

FIGS. 22A, 22B1, 22B2, 22C, and 22D illustrate a bendable secondarybattery;

FIGS. 23A and 23B illustrate a bendable secondary battery;

FIGS. 24A to 24H illustrate examples of electronic devices;

FIGS. 25A to 25C illustrate an example of an electronic device;

FIG. 26 illustrates examples of electronic devices;

FIGS. 27A to 27C illustrate examples of electronic devices;

FIG. 28 is a transmission electron microscope image of a positiveelectrode active material in Example 1;

FIGS. 29A1, 29A2, 29B1, 29B2, 29C1, and 29C2 are FFT images oftransmission electron microscope images of a positive electrode activematerial in Example 1;

FIGS. 30A1, 30A2, 30B1, 30B2, 30C1, and 30C2 are element mapping imagesof a positive electrode active material in Example 1;

FIGS. 31A1, 31A2, 31B1, 31B2, 31C1, and 31C2 are element mapping imagesof a positive electrode active material of a comparative example inExample 1;

FIG. 32 is a graph showing TEM-EDX line analysis results of a positiveelectrode active material in Example 1;

FIG. 33 is a graph showing charge and discharge characteristics of asecondary battery in Example 1;

FIG. 34 is a graph showing charge and discharge characteristics of asecondary battery of a comparative example in Example 1;

FIG. 35 is a graph showing cycle characteristics of a secondary batteryin Example 1;

FIG. 36 is a graph showing cycle characteristics of a secondary batteryin Example 1;

FIGS. 37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2, 37E1, and 37E2 areTEM-EDX plane analysis images of a comparative example in Example 2;

FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1, 38D2, 38E1, and 38E2 areTEM-EDX plane analysis images of a positive electrode active material inExample 2;

FIGS. 39A, 39B1, 39B2, 39C1, 39C2, 39D1, 39D2, 39E1, and 39E2 areTEM-EDX plane analysis images of a comparative example in Example 2;

FIGS. 40A, 40B1, 40B2, 40C1, 40C2, 40D1, 40D2, 40E1, and 40E2 areTEM-EDX plane analysis images of a positive electrode active material inExample 2;

FIGS. 41A and 41B are each a graph showing EDX point analysis results inExample 2;

FIGS. 42A and 42B are each a graph showing EDX point analysis results inExample 2;

FIG. 43 is a graph showing rate characteristics of a secondary batteryin Example 2;

FIG. 44 is a graph showing temperature characteristics of a secondarybattery in Example 2;

FIG. 45 is a graph showing cycle characteristics of a secondary batteryin Example 2;

FIGS. 46A and 46B are each a graph showing XPS analysis results of apositive electrode active material in Example 3;

FIGS. 47A and 47B are each a graph showing cycle characteristics of asecondary battery containing a positive electrode active material inExample 3;

FIG. 48 is a graph showing cycle characteristics of a secondary batterycontaining a positive electrode active material in Example 3;

FIG. 49 is a graph showing cycle characteristics of a secondary batterycontaining a positive electrode active material in Example 3;

FIGS. 50A to 50C are each a graph showing charge and dischargecharacteristics of a secondary battery containing a positive electrodeactive material in Example 3;

FIGS. 51A to 51C are SEM images of a positive electrode active materialin Example 4;

FIGS. 52A-1, 52A-2, 52B-1, 52B-2, 52C-1, and 52C-2 are SEM-EDX images ofa positive electrode active material in Example 4;

FIG. 53 is a process flow chart of Example 5;

FIG. 54 illustrates a spray dry apparatus in Example 5;

FIG. 55 is a TEM image showing one embodiment of the present inventionin Example 5;

FIG. 56 is a SEM image showing one embodiment of the present inventionin Example 5;

FIG. 57 is a SEM image showing a comparative example in Example 5; and

FIGS. 58A and 58B are cross-sectional views of an active material layercontaining a graphene compound as a conductive additive in Example 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details of the present invention canbe modified in various ways. In addition, the present invention shouldnot be construed as being limited to the description in the embodimentsgiven below.

Note that in drawings used in this specification, the sizes,thicknesses, and the like of components such as a positive electrode, anegative electrode, an active material layer, a separator, and anexterior body are exaggerated for simplicity in some cases. Therefore,the sizes of the components are not limited to the sizes in the drawingsand relative sizes between the components.

Note that in structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions are denoted by common reference numerals in differentdrawings, and descriptions thereof are not repeated. Further, the samehatching pattern is applied to portions having similar functions, andthe portions are not especially denoted by reference numerals in somecases.

In this specification and the like, the Miller index is used for theexpression of crystal planes and orientations. In the crystallography, asuperscript bar is placed over a number in the expression using theMiller index; however, in this specification and the like, crystalplanes and orientations are expressed by placing a minus sign (−) at thefront of a number instead of placing the bar over a number because ofexpression limitations. Furthermore, an individual direction which showsan orientation in crystal is denoted by “[ ]”, a set direction whichshows all of the equivalent orientations is denoted by “< >”, anindividual direction which shows a crystal plane is denoted by “( )”,and a set plane having equivalent symmetry is denoted by “{ }”. In thedrawings, the crystal planes and orientations are expressed by a numberwith a bar placed thereover, which is an original crystallographicexpression. Note that 1 Å is 10⁻¹⁰ m.

In this specification and the like, segregation refers to a phenomenonin which, in a solid made of a plurality of elements (e.g., A, B, andC), a certain element (for example, B) is non-uniformly distributed.

In this specification and the like, a layered rock-salt crystalstructure included in a composite oxide containing lithium and atransition metal refers to a crystal structure in which a rock-salt ionarrangement where cations and anions are alternately arranged isincluded and the lithium and the transition metal are regularly arrangedto form a two-dimensional plane, so that lithium can betwo-dimensionally diffused. Note that a defect such as a cation or anionvacancy can exist. In the layered rock-salt crystal structure, strictly,a lattice of a rock-salt crystal is distorted in some cases.

In this specification and the like, a rock-salt crystal structure refersto a structure in which cations and anions are alternately arranged.Note that a cation or anion vacancy may exist.

Anions of a layered rock-salt crystal and anions of a rock-salt crystaleach form a cubic closest packed structure (face-centered cubic latticestructure). When a layered rock-salt crystal and a rock-salt crystal arein contact with each other, there is a crystal plane at which directionsof cubic closest packed structures formed of anions are aligned witheach other. A space group of the layered rock-salt crystal is R-3m,which is different from a space group Fm-3m of a general rock-saltcrystal and a space group Fd-3m of a rock-salt crystal having thesimplest symmetry; thus, the Miller index of the crystal planesatisfying the above conditions in the layered rock-salt crystal isdifferent from that in the rock-salt crystal. In this specification, inthe layered rock-salt crystal and the rock-salt crystal, a state wherethe directions of the cubic closest packed structures formed of anionsare aligned with each other is referred to as a state where crystalorientations are substantially aligned with each other.

Whether the crystal orientations in two regions are aligned with eachother or not can be judged by a transmission electron microscope (TEM)image, a scanning transmission electron microscope (STEM) image, ahigh-angle annular dark field scanning transmission electron microscopy(HAADF-STEM) image, an annular bright-field scan transmission electronmicroscopy (ABF-STEM) image, and the like. X-ray diffraction, electrondiffraction, neutron diffraction, and the like can be used for judging.In the TEM image and the like, alignment of cations and anions can beobserved as repetition of bright lines and dark lines. When theorientations of cubic closest packed structures of the layered rock-saltcrystal and the rock-salt crystal are aligned with each other, a statewhere an angle between the repetition of bright lines and dark lines inthe layered rock-salt crystal and the repetition of bright lines anddark lines in the rock-salt crystal is less than or equal to 5°,preferably less than or equal to 2.5° is observed. Note that, in the TEMimage and the like, a light element such as oxygen or fluorine is notclearly observed in some cases; however, in such a case, alignment oforientations can be judged by arrangement of metal elements.

Furthermore, in this specification and the like, a state wherestructures of two-dimensional interfaces have similarity is referred toas “epitaxy”. Crystal growth in which structures of two-dimensionalinterfaces have similarity is referred to as “epitaxial growth”. Inaddition, a state where three-dimensional structures have similarity ororientations are crystallographically the same is referred to as“topotaxy”. Thus, in the case of topotaxy, when part of a cross sectionis observed, orientations of crystals in two regions (e.g., a regionserving as a base and a region formed through growth) are substantiallyaligned with each other.

Embodiment 1 [Structure of Positive Electrode Active Material]

First, a positive electrode active material 100, which is one embodimentof the present invention, is described with reference to FIGS. 1A to 1C.The positive electrode active material 100 refers to a substancecontaining a transition metal which can receive and release lithium ionelectrochemically. As illustrated in FIG. 1A, the positive electrodeactive material 100 includes a first region 101 inside and a secondregion 102 and a third region 103 in a superficial portion.

As illustrated in FIG. 1B, the second region 102 does not necessarilycover the entire first region 101. Similarly, the third region 103 doesnot necessarily cover the entire second region 102. In addition, thethird region 103 may be present in contact with the first region 101.

The thicknesses of the second region 102 and the third region 103 mayeach differ depending on the positions.

Furthermore, the third region 103 may be present inside the positiveelectrode active material 100. For example, in the case where the firstregion 101 is a polycrystal, the third region 103 may be present in thevicinity of a grain boundary. Furthermore, the third region 103 may bepresent in a portion which includes crystal defects or a crack portionin the positive electrode active material 100 or in the vicinitythereof. In FIG. 1B, parts of grain boundaries are shown by dottedlines. In this specification and the like, crystal defects refer todefects which can be observed from a TEM image and the like, that is, astructure in which another element enters crystal, a cavity, and thelike. In addition, a crack portion refers to a crack or a break formedin a particle like a crack portion 106 illustrated in FIG. 1C, forexample.

Similarly, as illustrated in FIG. 1B, the second region 102 may bepresent inside the positive electrode active material 100. For example,in the case where the first region 101 is a polycrystal, the secondregion 102 may be present in the vicinity of a grain boundary.Furthermore, the second region 102 may be present in a portion whichincludes crystal defects or a crack portion in the positive electrodeactive material 100 or in the vicinity thereof. Moreover, the thirdregion 103 and the second region 102 inside the positive electrodeactive material 100 may overlap.

<First Region 101>

The first region 101 contains a composite oxide of lithium and a firsttransition metal. In other words, the first region 101 contains lithium,a first transition metal, and oxygen.

The composite oxide of lithium and a first transition metal preferablyhas a layered rock-salt crystal structure.

As the first transition metal, only cobalt may be used, cobalt andmanganese may be used, or cobalt, manganese, and nickel may be used.

That is, the first region can include lithium cobalt oxide, lithiummanganese oxide, lithium nickel oxide, lithium cobalt oxide in whichmanganese is substituted for part of cobalt, lithiumnickel-manganese-cobalt oxide, or the like. In addition to thetransition metal, the first region 101 may include a metal other thanthe transition metal, such as aluminum.

The first region 101 serves as a region which particularly contributesto a charge and discharge reaction in the positive electrode activematerial 100. To increase capacity of a secondary battery containing thepositive electrode active material 100, the volume of the first region101 is preferably larger than those of the second region and the thirdregion.

A material having a layered rock-salt crystal structure has featuressuch as high discharge capacity and low resistance due to lithium thatcan be diffused two-dimensionally, which is preferably used for thefirst region 101. In addition, in the case where the first region 101has a layered rock-salt crystal structure, a segregation of arepresentative element such as magnesium, which is described later,tends to occur unexpectedly.

Note that the first region 101 may be a single crystal or a polycrystal.For example, the first region 101 may be a polycrystal in which anaverage crystallite size is greater than or equal to 280 nm and lessthan or equal to 630 nm. In the case of a polycrystal, a grain boundarycan be observed from the TEM or the like in some cases. In addition, theaverage of crystal grain sizes can be calculated from the half width ofXRD.

The polycrystal has a clear crystal structure; thus, a two-dimensionaldiffusion path of lithium ions can be sufficiently ensured. In addition,a polycrystal is easily produced as compared with a single crystal;thus, a polycrystal is preferably used for the first region 101.

Furthermore, the entire first region 101 does not necessarily have alayered rock-salt crystal structure. For example, part of the firstregion 101 may be amorphous or has another crystal structure.

<Second Region 102>

The second region 102 contains an oxide of a second transition metal. Inother words, the second region 102 contains a second transition metaland oxygen.

As the second transition metal, a non-stoichiometric metal is preferablyused. In other words, the second region 102 preferably includes anon-stoichiometric compound. For example, as the second transitionmetal, at least one of titanium, vanadium, manganese, iron, chromium,niobium, cobalt, zinc, zirconium, nickel, and the like can be used. Notethat the second transition metal is preferably an element different fromthat of the first transition metal.

In this specification and the like, a non-stoichiometric metal refers toa metal that can have a plurality of valences. In addition, anon-stoichiometric compound refers to a compound of a metal that canhave a plurality of valences and another element.

The second region 102 preferably has a rock-salt crystal structure.

The second region 102 serves as a buffer region which connects the firstregion 101 to the third region 103 which is described later. In thenon-stoichiometric compound, an interatomic distance can be changed inaccordance with a change in valence of a metal contained in thenon-stoichiometric compound. In addition, in the non-stoichiometriccompound, a cation or anion vacancy and dislocation (what is calledMagneli phase) are often formed. Thus, the second region 102, whichserves as a buffer region, can absorb a strain generated between thefirst region 101 and the third region 103.

Furthermore, the second region 102 may contain lithium in addition tothe second transition metal and oxygen. For example, lithium titanate orlithium manganite may be contained. Moreover, the second region 102 maycontain a representative element contained in the third region 103 whichis described later. The second region 102 that contains an elementcontained in the first region 101, such as lithium, and an elementcontained in the third region 103 is preferable because the secondregion 102 serves as a buffer region.

That is, the second region 102 can contain lithium titanate, titaniumoxide, vanadium oxide, manganese oxide, iron oxide, copper oxide,chromium oxide, niobium oxide, cobalt oxide, zinc oxide, or the like.

In addition, the second region 102 may contain the first transitionmetal. For example, the second transition metal may be present in partof a first transition metal site of the composite oxide including thefirst transition metal.

For example, in the case where the second transition metal is titanium,titanium may be present as titanium oxide (TiO₂) or lithium titanate(LiTiO₂) in the second region 102. Alternatively, in the second region102, titanium may be substituted for part of the first transition metalsite of the composite oxide of lithium and the first transition metal.

Moreover, the second region 102 may contain fluorine.

The second region 102 preferably has a crystal structure which is thesame as that of the third region 103 which is described later. In thiscase, orientations of crystals of the second region 102 and the thirdregion 103 are likely to be aligned with each other.

The second region 102 preferably has a rock-salt crystal structure;however, the entire second region 102 does not necessarily have arock-salt crystal structure. For example, the second region 102 may haveanother crystal structure such as a spinel crystal structure, an olivinecrystal structure, a corundum crystal structure, or a rutile crystalstructure.

Furthermore, a crystal structure may have a strain as long as astructure where six oxygen atoms are adjacent to cations is kept. Inaddition, a cation vacancy may be present in part of the second region102.

Moreover, part of the second region 102 may be amorphous.

When the thickness of the second region 102 is too small, the functionas the buffer region is degraded; however, when the thickness of thesecond region 102 is too large, the capacity might be decreased. Thus,the second region 102 is preferably provided in a range from the surfaceof the positive electrode active material 100 to a depth of 20 nm,preferably a depth of 10 nm, in a depth direction. The second transitionmetal may have a concentration gradient.

<Third Region 103>

The third region 103 contains a compound of representative elements. Acompound of representative elements is a stoichiometric compound. As thecompound of representative elements, a compound made of representativeelements which are electrochemically stable is preferable, and at leastone of magnesium oxide, calcium oxide, beryllium oxide, lithiumfluoride, and sodium fluoride can be used, for example.

The third region 103 is in contact with an electrolyte solution when thepositive electrode active material 100 is used in a secondary battery.Thus, for the third region 103, a material which is hardly changedelectrochemically in the process of charging and discharging and is noteasily transformed by contact with the electrolyte solution ispreferably used. The compound of representative elements which is astoichiometric compound and electrochemically stable is preferably usedfor the third region 103. The positive electrode active material 100includes the third region 103 in a superficial portion to improvestability in charging and discharging of the secondary battery. Here, astate where stability of a secondary battery is high refers to a statewhere the crystal structure of the composite oxide of lithium and thefirst transition metal contained in the first region 101 is more stable.Alternatively, it refers to a state where a change in capacity of thesecondary battery is small even if charging and discharging are repeatedor a state where a change in valence of a metal contained in thepositive electrode active material 100 is suppressed even after chargingand discharging are repeated.

The third region 103 may contain fluorine. In the case where the thirdregion 103 contains fluorine, fluorine may be substituted for someanions in the compound of the representative elements.

Fluorine is substituted for some anions in the compound of therepresentative elements, whereby diffusion properties of lithium can beimproved. Thus, the third region 103 is less likely to prevent chargingand discharging. In addition, when fluorine is present in a superficialportion of a positive electrode active material particle, corrosionresistance against a hydrofluoric acid generated by decomposition of anelectrolyte solution is increased in some cases.

Moreover, the third region 103 may include lithium, the first transitionmetal, and the second transition metal.

The compound of the representative elements contained in the thirdregion 103 preferably has a rock-salt crystal structure. When the thirdregion 103 has a rock-salt crystal structure, orientations of crystalsare likely to be aligned with those of the second region 102. Theorientations of crystals of the first region 101, the second region 102,and the third region 103 are substantially aligned with each other,whereby the second region 102 and the third region 103 can serve as amore stable coating layer.

However, the entire third region 103 does not necessarily have arock-salt crystal structure. For example, the third region 103 may haveanother crystal structure such as a spinel crystal structure, an olivinecrystal structure, a corundum crystal structure, or a rutile crystalstructure.

Furthermore, a crystal structure may have a strain as long as astructure where six oxygen atoms are adjacent to cations is kept. Inaddition, a cation vacancy may be present in part of the third region103.

Moreover, part of the third region 103 may be amorphous.

When the thickness of the third region 103 is too small, the function ofincreasing stability in charging and discharging is degraded; however,when the thickness of the third region 103 is too large, the capacitymight be decreased. Thus, the thickness of the third region 103 ispreferably greater than or equal to 0.5 nm and less than or equal to 50nm, further preferably greater than or equal to 0.5 nm and less than orequal to 2 nm.

In the case where the third region 103 contains fluorine, fluorine ispreferably present in a bonding state other than magnesium fluoride(MgF₂), lithium fluoride (LiF), and cobalt fluoride (CoF₂).Specifically, when an XPS analysis is performed on the vicinity of thesurface of the positive electrode active material 100, a peak positionof bonding energy with fluorine is preferably higher than or equal to682 eV and lower than or equal to 685 eV, further preferablyapproximately 684.3 eV. The bonding energy does not correspond to thoseof MgF₂, LiF, and CoF₂.

In this specification and the like, a peak position of bonding energywith an element in an XPS analysis refers to a value of bonding energyat which the maximum intensity of an energy spectrum is obtained in arange corresponding to bonding energy of the element.

In general, when charging and discharging are repeated, a side reactionoccurs in a positive electrode active material, for example, a firsttransition metal such as manganese, cobalt, or nickel is dissolved in anelectrolyte solution, oxygen is released, and a crystal structurebecomes unstable, so that the positive electrode active materialdeteriorates. However, the positive electrode active material 100 of oneembodiment of the present invention includes the second region 102serving as a buffer region and the third region 103 which iselectrochemically stable. Thus, the dissolution of the first transitionmetal can be effectively suppressed, and the crystal structure of thecomposite oxide of lithium and the transition metal contained in thefirst region 101 can be more stable. As a result, the cyclecharacteristics of the secondary battery including the positiveelectrode active material 100 can be significantly improved. Inaddition, in the case where charging and discharging are performed at avoltage higher than 4.3 V (vs. Li/Li⁺), in particular, a high voltage of4.5 V (vs. Li/Li⁺) or more, the structure of one embodiment of thepresent invention is significantly effective.

<Heteroepitaxial Growth and Topotaxy>

The second region 102 is preferably formed by heteroepitaxial growthfrom the first region 101. Furthermore, the third region 103 ispreferably formed by heteroepitaxial growth from the second region 102.A region formed by heteroepitaxial growth becomes topotaxy which hascrystal orientations substantially three-dimensionally aligned withthose of a region serving as a base. Thus, the first region 101, thesecond region 102, and the third region 103 can be topotaxy.

When the crystal orientations of the first region 101, the second region102, and the third region 103 are substantially aligned with each other,the second region 102 and the third region 103 serve as a coating layerwhich has a stable bond with the first region 101. As a result, thepositive electrode active material 100 including a strong coating layercan be provided.

Since the second region 102 and the third region 103 have a stable bondwith the first region 101, when the positive electrode active material100 is used for the secondary battery, a change of the crystal structurein the first region 101 which is caused by charging and discharging canbe effectively suppressed. Even when lithium is released from the firstregion 101 due to charging, the coating layer having a stable bond cansuppress release of cobalt and oxygen from the first region 101.Furthermore, a chemically stable material can be used for a region incontact with the electrolyte solution. Thus, a secondary battery havingexcellent cycle characteristics can be provided.

<Degree of Mismatch Between Regions>

To perform heteroepitaxial growth, the degree of a mismatch betweencrystals in a region serving as a base and crystals on which crystalgrowth is performed is important.

In this specification and the like, the degree of a mismatch f isdefined by the following Formula 1. The average of the nearest neighbordistances between oxygen and cations of the crystals in the regionserving as a base is represented by a, and the average of the naturalnearest neighbor distances between anions and cations of the crystals onwhich crystal growth is performed is represented by b.

[Formula 1]

f=|b−a/a|(Formula 1)

To perform heteroepitaxial growth, the degree of a mismatch f betweencrystals in a region serving as a base and crystals on which crystalgrowth is performed needs to be less than or equal to 0.12. To performmore stable heteroepitaxial growth to form a layered shape, the degreeof a mismatch f is preferably less than or equal to 0.08, furtherpreferably less than or equal to 0.04.

Thus, materials of the first region 101 and the second region 102 arepreferably selected so that the degree of a mismatch f between thelayered rock-salt crystal structure in the first region 101 and therock-salt crystal structure in the second region 102 is less than orequal to 0.12.

Furthermore, materials of the second region 102 and the third region 103are preferably selected so that the degree of a mismatch f between therock-salt crystal structure in the second region 102 and the rock-saltcrystal structure in the third region 103 is less than or equal to 0.12.

Examples of materials and crystal planes of the first region 101, thesecond region 102, and the third region 103 are shown below, whichsatisfy the above-described conditions: the degree of a mismatch fbetween the layered rock-salt crystal structure in the first region 101and the rock-salt crystal structure in the second region 102 is lessthan or equal to 0.12; and the degree of a mismatch f between therock-salt crystal structure in the second region 102 and the rock-saltcrystal structure in the third region 103 is less than or equal to 0.12.

Example 1: Lithium Cobaltate, Lithium Titanate, and Magnesium Oxide

First, FIGS. 2A and 2B and FIG. 3 show an example in which the firsttransition metal is cobalt, the first region 101 contains lithiumcobaltate having a layered rock-salt crystal structure, the secondtransition metal is titanium, the second region 102 contains lithiumtitanate having a rock-salt crystal structure, and the compound of therepresentative elements in the third region 103 is magnesium oxidehaving a rock-salt crystal structure.

FIG. 2A illustrates a model of a layered rock-salt crystal structure (aspace group R-3mH) of lithium cobaltate (LiCoO₂), a model of a rock-saltcrystal structure (a space group Fd-3mZ) of lithium titanate (LiTiO₂),and a model of a rock-salt crystal structure (a space group Fd-3mZ) ofmagnesium oxide. FIG. 2A illustrates models each of which is seen fromthe b-axis direction.

From FIG. 2A, it is not seen that the layered rock-salt crystals and therock-salt crystals make topotaxy. Then, here, the layered rock-saltcrystals are seen from a different direction (e.g., a directionindicated by an arrow in FIG. 2A). FIG. 2B illustrates a model of thelayered rock-salt crystals seen from the <1-1-4> plane direction andmodels of the rock-salt crystals seen from the <100> plane direction.

As illustrated in FIG. 2B, when the layered rock-salt crystals are seenfrom the <1-1-4> plane direction, atomic arrangement of the layeredrock-salt crystals is highly similar to those of the rock-salt crystalsseen from the <100> plane direction. In addition, the nearest neighbordistances between metal and oxygen have similar values. For example, inthe layered rock-salt lithium cobaltate, a distance between Li and O is2.089 Å and a distance between Co and O is 1.925 Å. In the rock-saltlithium titanate, a distance between Li and O is 2.138 Å and a distancebetween Ti and O is 2.051 Å. In the rock-salt magnesium oxide, adistance between Mg and O is 2.106 Å.

Then, with reference to FIG. 3, the degree of a mismatch between theregions when the (1-1-4) crystal plane of the layered rock-salt crystaland the {100} crystal plane of the rock-salt crystal are in contact witheach other is described.

As illustrated in FIG. 3, a distance between metals through oxygen in a(1-1-4) crystal plane 101 p (1-1-4) of lithium cobaltate having thelayered rock-salt crystal structure in the first region 101 is 4.01 Å.Furthermore, a distance between metals through oxygen in a {100} crystalplane 102 p {100} of lithium titanate having the rock-salt crystalstructure in the second region 102 is 4.19 Å. Thus, the degree of amismatch f between the crystal plane 101 p (1-1-4) and the crystal plane102 p {100} is 0.04.

In addition, a distance between metals through oxygen in a {100} crystalplane 103 p {100} of magnesium oxide having the rock-salt crystalstructure in the third region 103 is 4.21 Å. Thus, the degree of amismatch f between the crystal plane 102 p {100} and the crystal plane103 p {100} is 0.02.

In this manner, the degree of a mismatch between the first region 101and the second region 102 and the degree of a mismatch between thesecond region 102 and the third region 103 are sufficiently small; thus,the first region 101, the second region 102, and the third region 103can be topotaxy.

Although not illustrated in FIG. 3, if the crystal plane 101 p (1-1-4)in the first region 101 and the crystal plane 103 p {100} in the thirdregion 103 are in contact with each other, the degree of a mismatch f is0.05. That is, owing to the second region 102, the degree of a mismatchcan be small. Moreover, with the second region 102 that is anon-stoichiometric transition metal oxide, the first region 101, thesecond region 102, and the third region 103 can be more stable topotaxy.Thus, the second region 102 and the third region 103 can serve as acoating layer having a stable bond with the first region 101.

In this embodiment, the (1-1-4) plane of the layered rock-salt crystaland the {100} plane of the rock-salt crystal are in contact with eachother; however, one embodiment of the present invention is not limitedthereto as long as crystal planes which can be topotaxy are in contactwith each other.

Example 2: Lithium Cobaltate, Manganese Oxide, and Calcium Oxide

Next, an example in which the first transition metal is cobalt, thefirst region 101 contains lithium cobaltate having a layered rock-saltcrystal structure, the second transition metal is manganese, the secondregion 102 contains manganese oxide having a rock-salt crystalstructure, and the compound of the representative elements in the thirdregion 103 is calcium oxide having a rock-salt crystal structure isshown.

Also in this case, as in FIGS. 2A and 2B and FIG. 3, when the layeredrock-salt crystals are seen from the <1-1-4> plane direction, atomicarrangement of the layered rock-salt crystals in the first region 101 ishighly similar to those of the rock-salt crystals in the second region102 and the third region 103 seen from the <100> plane direction.

The degree of a mismatch between the regions when the (1-1-4) crystalplane of the layered rock-salt crystal and the {100} crystal plane ofthe rock-salt crystal are in contact with each other is described. Adistance between metals through oxygen in a crystal plane (1-1-4) oflithium cobaltate having the layered rock-salt crystal structure in thefirst region 101 is 4.01 Å. Furthermore, a distance between metalsthrough oxygen in a crystal plane {100} of manganese oxide having therock-salt crystal structure in the second region 102 is 4.45 Å. Thus,the degree of a mismatch f between the crystal plane (1-1-4) in thefirst region 101 and the crystal plane {100} in the second region 102 is0.11.

In addition, a distance between metals through oxygen in a crystal plane{100} of calcium oxide having the rock-salt crystal structure in thethird region 103 is 4.82 Å. Thus, the degree of a mismatch f between thecrystal plane {100} in the second region 102 and the crystal plane {100}in the third region 103 is 0.08.

In this manner, the degree of a mismatch between the first region 101and the second region 102 and the degree of a mismatch between thesecond region 102 and the third region 103 are sufficiently small; thus,the first region 101, the second region 102, and the third region 103can be topotaxy.

If the crystal plane (1-1-4) in the first region 101 and the crystalplane {100} in the third region 103 are in contact with each other, thedegree of a mismatch f is 0.20; thus, it is difficult to perform theheteroepitaxial growth. That is, owing to the second region 102, theheteroepitaxial growth from the first region to the third region can beperformed. Thus, the second region 102 and the third region 103 canserve as a coating layer having a stable bond with the first region 101.

Example 3: Lithium Nickel-Manganese-Cobalt Oxide, Manganese Oxide, andCalcium Oxide

Next, an example in which the first transition metals are nickel,manganese, and cobalt, the first region 101 contains lithiumnickel-manganese-cobalt oxide (LiNi_(0.33)Co_(0.33)Mn_(0.3302)) having alayered rock-salt crystal structure, the second transition metal ismanganese, the second region 102 contains manganese oxide having arock-salt crystal structure, and the compound of the representativeelements in the third region 103 is calcium oxide having a rock-saltcrystal structure is shown.

Also in this case, as illustrated in FIGS. 2A and 2B and FIG. 3, whenthe layered rock-salt crystals are seen from the <1-1-4> planedirection, atomic arrangement of the layered rock-salt crystals ishighly similar to those of the rock-salt crystals seen from the <100>plane direction. The degree of a mismatch between the regions when the(1-1-4) crystal plane of the layered rock-salt crystal and the {100}crystal plane of the rock-salt crystal are in contact with each other isdescribed.

A distance between metals through oxygen in a crystal plane (1-1-4) oflithium nickel-manganese-cobalt oxide having the layered rock-saltcrystal structure in the first region 101 is 4.07 Å. Furthermore, adistance between metals through oxygen in a crystal plane {100} ofmanganese oxide having the rock-salt crystal structure in the secondregion 102 is 4.45 Å. Thus, the degree of a mismatch f between thecrystal plane (1-1-4) in the first region 101 and the crystal plane{100} in the second region 102 is 0.09.

In addition, a distance between metals through oxygen in a crystal plane{100} of calcium oxide having the rock-salt crystal structure in thethird region 103 is 4.82 Å. Thus, the degree of a mismatch f between thecrystal plane {100} in the second region 102 and the crystal plane {100}in the third region 103 is 0.08.

In this manner, the degree of a mismatch between the first region 101and the second region 102 and the degree of a mismatch between thesecond region 102 and the third region 103 are sufficiently small; thus,the first region 101, the second region 102, and the third region 103can be topotaxy.

If the crystal plane (1-1-4) in the first region 101 and the crystalplane {100} in the third region 103 are in contact with each other, thedegree of a mismatch f is 0.18; thus, it is difficult to perform theheteroepitaxial growth. That is, the second region 102 is provided,whereby the heteroepitaxial growth from the first region to the thirdregion can be performed. Thus, the second region 102 and the thirdregion 103 can serve as a coating layer having a stable bond with thefirst region 101.

<Boundaries Between Regions>

As described above, the first region 101, the second region 102, and thethird region 103 have different compositions. The element contained ineach region has a concentration gradient in some cases. For example, thesecond transition metal in the second region 102 may have aconcentration gradient. In addition, the third region 103 may have aconcentration gradient of a representative element because arepresentative element preferably segregates in the third region 103 asdescribed later. Thus, the boundaries between the regions are not clearin some cases.

The difference of compositions of the first region 101, the secondregion 102, and the third region 103 can be observed using a TEM image,a STEM image, fast Fourier transform (FFT) analysis, energy dispersiveX-ray spectrometry (EDX), an analysis in the depth direction bytime-of-flight secondary ion mass spectrometry (ToF-SIMS), X-rayphotoelectron spectroscopy (XPS), Auger electron spectroscopy, thermaldesorption spectroscopy (TDS), or the like.

For example, in the TEM image and the STEM image, difference ofconstituent elements is observed as difference of brightness; thus,difference of constituent elements of the first region 101, the secondregion 102, and the third region 103 can be observed. Furthermore, alsoin plane analysis of EDX (e.g., element mapping), it can be observedthat the first region 101, the second region 102, and the third region103 contain different elements.

By line analysis of EDX and analysis in the depth direction usingToF-SIMS, a peak of concentration of each element contained in the firstregion 101, the second region 102, and the third region 103 can bedetected.

However, clear boundaries between the first region 101, the secondregion 102, and the third region 103 are not necessarily observed by theanalyses.

In this specification and the like, the third region 103 that is presentin a superficial portion of the positive electrode active material 100refers to a region from the surface of the positive electrode activematerial 100 to a region where a concentration of a representativeelement such as magnesium which is detected by analysis in the depthdirection is ⅕ of a peak. As the analysis in the depth direction, theline analysis of EDX, analysis in the depth direction using ToF-SIMS, orthe like, which is described above, can be used.

Furthermore, a peak of a concentration of a representative element ispreferably present in a region from the surface of the positiveelectrode active material 100 to a depth of 3 nm toward the center,further preferably to a depth of 1 nm, and still further preferably to adepth of 0.5 nm.

Although the depth at which the concentration of the representativeelement becomes ⅕ of the peak is different depending on themanufacturing method, in the case of a manufacturing method describedlater, the depth is approximately 2 nm to 5 nm from the surface of thepositive electrode active material.

The third region 103 that is present inside the first region 101 in thevicinity of a grain boundary, a crystal defect, or the like also refersto a region where a concentration of a representative element which isdetected by analysis in the depth direction is higher than or equal to ⅕of a peak.

A distribution of fluorine in the positive electrode active material 100preferably overlaps with a distribution of the representative element.Thus, fluorine also has a concentration gradient, and a peak of aconcentration of fluorine is preferably present in a region from thesurface of the positive electrode active material 100 to a depth of 3 nmtoward the center, further preferably to a depth of 1 nm, and stillfurther preferably to a depth of 0.5 nm.

In this specification and the like, the second region 102 that ispresent in a superficial portion of the positive electrode activematerial 100 refers to a region where a concentration of the secondtransition metal which is detected by analysis in the depth direction ishigher than or equal to ½ of a peak. The second region 102 that ispresent inside the first region 101 in the vicinity of a grain boundary,a crystal defect, or the like also refers to a region where aconcentration of the second transition metal which is detected byanalysis in the depth direction is higher than or equal to ½ of a peak.As the analysis method, the line analysis of EDX, analysis in the depthdirection using ToF-SIMS, or the like, which is described above, can beused.

Thus, the third region 103 and the second region 102 overlap in somecases. Note that the third region 103 is preferably present in a regioncloser to the surface of the positive electrode active material particlethan the second region 102 is. In addition, the peak of theconcentration of the representative element is preferably present in aregion closer to the surface of the positive electrode active materialparticle than the peak of the concentration of the second transitionmetal is.

The peak of the second transition metal is preferably present in aregion from a depth of 0.2 nm or more to a depth of 10 nm or less fromthe surface of the positive electrode active material 100 toward thecenter, further preferably in a region from a depth of 0.5 nm or more toa depth of 3 nm or less.

The measurement range of the XPS is from the surface of the particle ofthe positive electrode active material 100 to a region at a depth ofapproximately 5 nm. Thus, a concentration of an element present at adepth of approximately 5 nm from the surface can be analyzedquantitatively. Thus, the concentration of elements in the third region103 and the second region 102 present at a depth of approximately 5 nmfrom the surface can be analyzed quantitatively.

When the surface of the positive electrode active material 100 issubjected to the XPS analysis and the concentration of the firsttransition metal is defined as 1, a relative value of the concentrationof the second transition metal is preferably greater than or equal to0.05 and less than or equal to 0.4, further preferably greater than orequal to 0.1 and less than or equal to 0.3. In addition, a relativevalue of the concentration of the representative element is preferablygreater than or equal to 0.4 and less than or equal to 1.5, furtherpreferably greater than or equal to 0.45 and less than or equal to 1.00.Furthermore, a relative value of the concentration of fluorine ispreferably greater than or equal to 0.05 and less than or equal to 1.5,further preferably greater than or equal to 0.3 and less than or equalto 1.00.

Note that, as described above, elements contained in the first region101, the second region 102, and the third region 103 may each have aconcentration gradient; thus, the first region 101 may contain theelement in the second region 102 or the third region 103, such asfluorine. Similarly, the third region 103 may contain the element in thefirst region 101 or the second region 102. In addition, the first region101, the second region 102, and the third region 103 may each containanother element such as carbon, sulfur, silicon, sodium, calcium,chlorine, or zirconium.

[Particle Diameter]

If the particle diameter of the positive electrode active material 100is too large, diffusion of lithium is difficult, whereas if the particlediameter is too small, it is difficult to maintain a crystal structuredescribed later. Thus, D50 (also referred to as a median diameter) ispreferably 5 μm or more and 100 μm or less, and further preferably 10 μmor more and 70 μm or less. In the case where the coating film is formedon the surface of the positive electrode active material 100 by a spraydry apparatus in a later step, it is preferable that the nozzle diameterand the maximum particle diameter of the positive electrode activematerial 100 be substantially the same. When the particle diameter isless than 5 μm and a spray dry apparatus having a nozzle diameter of 20μm is used, secondary particles are covered collectively, which leads toa decrease in coverage.

To increase the density of the positive electrode active material layer,it is effective to mix large particles (the longest portion isapproximately 20 μm or more and 40 μm or less) and small particles (thelongest portion is approximately 1 μm) and embed spaces between thelarge particles with the small particles. Thus, there may be two peaksof particle size distribution.

The particle size of the positive electrode active material isinfluenced not only by the particle sizes of starting materials but alsoby a ratio between lithium and the first transition metal (hereinafterexpressed as a ratio of Li to the first transition metal) which arecontained in the starting material.

In the case where the particle size of the starting material is small,the grain growth needs to be performed at the time of baking so that thegrain size of the positive electrode active material is in theabove-described preferred range.

To promote the grain growth at the time of baking, it is effective tomake the ratio of Li to the first transition metal of the startingmaterial larger than 1, that is, to make the amount of lithium a littlelarger. For example, when the ratio of Li to the first transition metalis approximately 1.06, a positive electrode active material in which D50is larger than or equal to 15 μm is easily obtained. Note that, asdescribed later, lithium may be lost to the outside of a system in theformation process of the positive electrode active material; thus, theratio between lithium and the first transition metal of the obtainedpositive electrode active material does not agree with the ratio betweenlithium and the first transition metal of the starting material in somecases.

However, if the amount of lithium is too large to make the particle sizebe in the preferred range, the capacity retention rate of a secondarybattery containing the positive electrode active material might bedecreased.

Then, the present inventors found that with the second region 102containing the second transition metal in the superficial portion, theparticle size can be in the preferred range by control of the ratio ofLi to the first transition metal and a positive electrode activematerial having high capacity retention rate can be formed.

In the positive electrode active material of one embodiment of thepresent invention including a region containing the second transitionmetal in the superficial portion, the ratio of Li to the firsttransition metal in the starting material is preferably greater than orequal to 1.00 and less than or equal to 1.07, further preferably greaterthan or equal to 1.03 and less than or equal to 1.06.

[Formation of Second Region]

The second region 102 can be formed by coating particles of thecomposite oxide of lithium and the first transition metal with amaterial containing the second transition metal.

As the coating method of the material containing the second transitionmetal, a liquid phase method such as a sol-gel method, a solid phasemethod, a sputtering method, an evaporation method, a chemical vapordeposition (CVD) method, a pulsed laser deposition (PLD) method, or thelike can be used. In this embodiment, the case where the sol-gel methodwhich can be performed with a uniform coverage under an atmosphericpressure is used is described.

<Sol-Gel Method>

A method for forming a material containing the second transition metalusing a sol-gel method is described with reference to FIGS. 4A-1, 4A-2,4A-3, 4B, 4C, 4D-1, and 4D-2.

First, an alkoxide of the second transition metal is dissolved inalcohol.

FIG. 4A-1 shows a general formula of the alkoxide of the secondtransition metal. In the formula of FIG. 4A-1, M2 indicates the alkoxideof the second transition metal. R represents an alkyl group having 1 to18 carbon atoms or a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Although FIG. 4A-1 shows the general formula in whichthe second transition metal has a valence of 4, one embodiment of thepresent invention is not limited thereto. The second transition metalmay have a valence of 2, a valence of 3, a valence of 5, a valence of 6,or a valence of 7. In this case, the alkoxide of the second transitionmetal includes an alkoxy group corresponding to the valence of thesecond transition metal.

FIG. 4A-2 shows a general formula of the titanium alkoxide used whentitanium is used as the second transition metal. R in FIG. 4A-2represents an alkyl group having 1 to 18 carbon atoms or a substitutedor unsubstituted aryl group having 6 to 13 carbon atoms.

As the titanium alkoxide, tetramethoxytitanium, tetraethoxytitanium,tetra-n-propoxytitanium, tetra-i-propoxytitanium (also referred to astetraisopropyl orthotitanate, titanium (IV) isopropoxide, titaniumtetraisopropoxide (IV), TTIP, and the like), tetra-n-butoxytitanium,tetra-i-butoxytitanium, tetra-sec-butoxytitanium,tetra-t-butoxytitanium, or the like can be used.

FIG. 4A-3 shows a chemical formula of titanium (IV) isopropoxide (TTIP)described in a formation method below, which is a kind of titaniumalkoxide.

As a solvent in which the alkoxide of the second transition metal isdissolved, an alcohol such as methanol, ethanol, propanol, 2-propanol,butanol, or 2-butanol is preferably used.

Next, particles of composite oxide of lithium, a transition metal,magnesium, and fluorine are mixed into the alcohol solution of thealkoxide of the second transition metal and stirred in an atmospherecontaining water vapor.

When the solution is put in an atmosphere containing H₂O, hydrolysis ofwater and an alkoxide of the second transition metal occurs as in FIG.4B. Then, as in FIG. 4C, dehydration condensation occurs between theproducts of FIG. 4B. When the hydrolysis of FIG. 4B and the condensationreaction of FIG. 4C occur repeatedly, a sol of an oxide of the secondtransition metal is generated. This reaction also occurs on a particle110 of the composite oxide as in FIGS. 4D-1 and 4D-2, and a layercontaining the second transition metal is formed on the surface of theparticle 110.

After that, the particle 110 is collected, and the alcohol is vaporized.The details of the formation method are described later.

Note that in this embodiment, an example in which the particles of thecomposite oxide of lithium, the first transition metal, therepresentative element, and fluorine are coated with the materialcontaining the second transition metal before the particles are appliedto a positive electrode current collector is described; however, oneembodiment of the present invention is not limited thereto. After thepositive electrode active material layer including the particles of thecomposite oxide of lithium, the first transition metal, therepresentative element, and fluorine is formed on the positive electrodecurrent collector, the positive electrode current collector and thepositive electrode active material layer may be both soaked into analkoxide solution of the second transition metal to be coated with thematerial containing the second transition metal.

[Segregation of Third Region]

The third region 103 can be formed by a sputtering method, a solid phasemethod, a liquid phase method such as a sol-gel method, or the like.However, the present inventors found that when a source of arepresentative element such as magnesium and a source of fluorine aremixed with a material of the first region 101 and then the mixture isheated, the representative element is segregated on a superficialportion of the positive electrode active material particle to form thethird region 103. In addition, they found that with the third region 103formed in this manner, the positive electrode active material 100 hasexcellent cycle characteristics.

In the case where the third region 103 is formed through the heating asdescribed above, the heating is preferably performed after the particleof the composite oxide is coated with the material containing the secondtransition metal. This is because even after the particle is coated withthe material containing the second transition metal, the representativeelement such as magnesium is unexpectedly segregated on the surface ofthe particle when the heating is performed.

Segregation models of the representative element are described withreference to FIGS. 5A to 5C and FIGS. 6A to 6D. It is probable that thesegregation model of the representative element such as magnesium isslightly different in accordance with the ratio between lithium and thefirst transition metal contained in a starting material. Then, asegregation model in which the ratio of Li to the first transition metalin the starting material is less than 1.03, that is, the amount oflithium is small, is described with reference to FIGS. 5A to 5C. Inaddition, a segregation model in which the ratio of Li to the firsttransition metal in the starting material is greater than or equal to1.03, that is, the amount of lithium is large, is described withreference to FIGS. 6A to 6D. In these segregation models in FIGS. 5A to5C and FIGS. 6A to 6D, the first transition metal is cobalt, the secondtransition metal is titanium, and the representative element ismagnesium.

FIG. 5A illustrates a model diagram of the vicinity of the surface ofthe particle 110 of the composite oxide containing lithium, cobalt,magnesium, and fluorine, which is formed at a ratio of Li to Co in thestarting material of less than 1.03. A region 111 in the drawingscontains lithium, cobalt, magnesium, and fluorine, and lithium cobaltate(LiCoO₂) is a main component of the region 111. Lithium cobaltate has alayered rock-salt structure.

It is generally known that, at the time of synthesizing particles of thecomposite oxide containing lithium, cobalt, magnesium, and fluorine,lithium partly moves outside a system (a particle on which lithium isformed). This is because lithium is volatilized at the time of baking,lithium is eluted to a solvent at the time of mixing the startingmaterial, and the like. Thus, the ratio of Li to Co in the particle 110of the composite oxide containing lithium, cobalt, magnesium, andfluorine becomes smaller than the ratio of Li to Co in the startingmaterial in some cases.

When the ratio of Li to Co in the starting material is less than 1.03,on the surface of the particle 110, lithium is released from the lithiumcobaltate and cobalt oxide is easily generated. Thus, as illustrated inFIG. 5A, the surface of the particle 110 of the composite oxide iscovered with a cobalt oxide (CoO) layer 114 in some cases.

The cobalt oxide has a rock-salt crystal structure. Thus, in theparticle 110 in FIG. 5A, the cobalt oxide layer 114 having a rock-saltcrystal structure is provided over and in contact with the region 111containing lithium cobaltate having a layered rock-salt crystalstructure in some cases.

The particle 110 is coated with a material containing titanium by asol-gel method or the like. FIG. 5B illustrates a state where theparticle 110 is coated with a layer 112 containing titanium by a sol-gelmethod. At the stage of FIG. 5B, the layer 112 containing titanium is agel of titanium oxide; thus, the crystallinity is low.

Next, the particle 110 coated with the layer 112 containing titanium isheated. Although the details of the heating conditions are describedlater, for example, FIG. 5C illustrates a state where the particle 110is heated in an oxygen atmosphere at 800° C. for two hours to form thepositive electrode active material 100, which is one embodiment of thepresent invention. By heating, titanium in the layer 112 containingtitanium is diffused into the inside of the particle 110. At the sametime, magnesium and fluorine contained in the region 111 are segregatedon the surface of the particle 110.

As described above, on the surface of the particle 110, cobalt oxidehaving a rock-salt structure is present. In addition, magnesium oxidealso has a rock-salt crystal structure. Thus, it is probable thatmagnesium is more stable in the state of being present as magnesiumoxide on the surface of the particle 110 as compared with the state ofbeing present inside the particle 110. That could be why magnesium issegregated on the surface of the particle 110 when the particle 110 isheated.

Moreover, it is considered that fluorine contained in the startingmaterial promotes the segregation of magnesium.

Fluorine has higher electronegativity than oxygen. Thus, it is probablethat even in a stable compound such as magnesium oxide, when fluorine isadded, uneven charge distribution occurs and a bond between magnesiumand oxygen is weakened. Furthermore, it is probable that fluorine issubstituted for oxygen in the magnesium oxide, whereby magnesium easilymoves around the substituted fluorine.

Moreover, this can also be described from a phenomenon in which amelting point of a mixture decreases. When magnesium oxide (meltingpoint: 2852° C.) and lithium fluoride (melting point: 848° C.) are addedat the same time, the melting point of the magnesium oxide is lowered.It is considered that the melting point is lowered, whereby magnesiumeasily moves at the time of heating, and the segregation of magnesiumeasily occurs.

Lastly, the third region 103 becomes a solid solution of cobalt oxideand magnesium oxide which has a rock-salt crystal structure.Furthermore, fluorine is probably substituted for part of oxygencontained in the cobalt oxide and the magnesium oxide.

Cobalt sites of lithium cobaltate are substituted for part of thediffused titanium and lithium titanate is substituted for another partof the diffused titanium. The second region 102 after the heatingcontains lithium titanate having a rock-salt crystal structure.

The first region 101 after the heating contains lithium cobaltate havinga layered rock-salt crystal structure.

Next, the case where the ratio of Li to Co in the starting material islarger than or equal to 1.03 is described with reference to FIGS. 6A to6D. FIG. 6A illustrates a model diagram of the vicinity of the surfaceof the particle 120 of the composite oxide containing lithium, cobalt,magnesium, and fluorine, which is formed at a ratio of Li to Co in thestarting material of greater than or equal to 1.03. A region 121 in thedrawings contains lithium, cobalt, magnesium, and fluorine.

Since the particle 120 in FIG. 6A contains a sufficient amount oflithium, even when lithium is released from the particle 120 at the timeof baking the particle 120 of the composite oxide of lithium, cobalt,magnesium, and fluorine or the like, lithium is diffused from the insideof the particle 120 to the surface thereof to compensate; as a result, acobalt oxide layer is not easily formed on the surface.

FIG. 6B illustrates a state where the particle 120 in FIG. 6A is coatedwith a layer 122 containing titanium by a sol-gel method. At the stageof FIG. 6B, the layer 122 containing titanium is a gel of titaniumoxide; thus, the crystallinity is low.

FIG. 6C illustrates the state where the particle 120 coated with thelayer 122 containing titanium in FIG. 6B starts to be heated. Byheating, titanium in the layer 122 containing titanium is diffused intothe inside of the particle 110. The diffused titanium is bonded withlithium contained in the region 121 to become lithium titanate, and alayer 125 containing the lithium titanate is formed.

Since lithium is bonded with titanium to form lithium titanate, theamount of lithium is relatively insufficient at the surface of theparticle 120. Thus, it is probable that, as illustrated in FIG. 6C, acobalt oxide layer 124 is temporarily formed on the surface of theparticle 120.

FIG. 6D illustrates the state where the state of FIG. 6C is sufficientlyheated to form the positive electrode active material 100, which is oneembodiment of the present invention. It is considered that, since thecobalt oxide layer 124 having a rock-salt crystal structure is presenton the surface, magnesium is more stable in the state of being presentas magnesium oxide on the surface of the particle 120 as compared withthe state of being present inside the particle 120. As in the case ofFIGS. 5A to 5C, fluorine promotes the segregation of magnesium.

Thus, as illustrated in FIG. 6D, magnesium and fluorine contained in theregion 121 are segregated on the surface to be the third region 103 withthe cobalt oxide.

In this manner, the positive electrode active material 100, whichincludes the third region 103 containing magnesium oxide and cobaltoxide, the second region 102 containing lithium titanate, and the firstregion 101 containing lithium cobaltate, is formed.

Note that in the case where the representative element is segregated byheating, when the composite oxide containing lithium and the firsttransition metal included in the first region 101 is a polycrystal orhas crystal defects, the representative element can be segregated notonly in the superficial portion but also in the vicinity of a grainboundary of the composite oxide containing lithium and the firsttransition metal or in the vicinity of crystal defects thereof. Therepresentative element segregated in the vicinity of a grain boundary orin the vicinity of crystal defects can contribute to further improvementin stability of the crystal structure of the composite oxide containinglithium and the first transition metal included in the first region 101.

When the composite oxide containing lithium and the first transitionmetal included in the first region 101 includes a crack portion, therepresentative element is also segregated in the crack portion byheating. In addition, not only the representative element but also thesecond transition metal can be segregated. The crack portion is incontact with the electrolyte solution like the surface of the particle.Thus, the representative element and the second transition metal aresegregated in the crack portion, and the third region 103 and the secondregion 102 are generated, whereby a chemically stable material can beused for the region in contact with the electrolyte solution. As aresult, a secondary battery having excellent cycle characteristics canbe provided.

The ratio between a representative element (T) and fluorine (F) in astarting material is preferably in a range of T:F=1:x (1.5≤x≤4) (atomicratio) because the segregation of the representative element effectivelyoccurs. Further preferably, the ratio between T and F is approximately1:2 (atomic ratio).

Since the third region 103 formed by segregation is formed by epitaxialgrowth, orientations of crystals in the second region 102 and the thirdregion 103 are partly and substantially aligned with each other in somecases. That is, the second region 102 and the third region 103 becometopotaxy in some cases. When the orientations of crystals in the secondregion 102 and the third region 103 are substantially aligned with eachother, these regions can serve as a more favorable coating layer.

However, not all of the representative elements such as magnesium whichis added as a starting material need not be segregated in the thirdregion 103. For example, the first region 101 may contain a small amountof representative element such as magnesium.

<Fourth Region 104>

In addition, as illustrated in FIG. 1C, the positive electrode activematerial 100 may include a fourth region 104 on the third region 103.Furthermore, when the positive electrode active material 100 contains adefect such as a crack portion 106, the fourth region 104 may be presentto embed the defect such as the crack portion 106.

The fourth region 104 contains some elements contained in the secondregion 102 and the third region 103. For example, the fourth region 104contains the second transition metal and the representative element.

The fourth region 104 may have a projection, a stripe shape, or alayered shape. The fourth region 104 is formed using the secondtransition metal and the representative element not contained in thesecond region 102 or the third region 103 of the second transitionmetals and the representative elements contained in the startingmaterial and the like. That is, with the fourth region 104, the amountof the second transition metal and the representative element containedin the second region 102 and the third region 103 can be kept in anappropriate range and the crystal structures of the second region 102and the third region 103 can be stabilized in some cases. Moreover, withthe fourth region 104, the defect such as the crack portion 106 includedin the positive electrode active material 100 can be repaired.

The presence of the fourth region 104 and the shape of the fourth region104 can be observed by a scanning electron microscope (SEM) or the like.Elements contained in the fourth region 104 can be analyzed by SEM-EDXor the like.

[Method for Forming Positive Electrode Active Material]

Next, an example of a method for forming the positive electrode activematerial 100, which is one embodiment of the present invention, isdescribed.

<Step 11: Preparation of Starting Materials>

First, starting materials are prepared. From the starting materialsprepared in this process, the first region 101 and the third region 103are formed finally.

As materials of lithium and the first transition metal contained in thefirst region 101, a source of lithium and a source of the firsttransition metal are prepared. In addition, as materials of the compoundof the representative elements contained in the third region 103, asource of the representative element is prepared.

In addition to these sources, a source of fluorine is preferablyprepared. Fluorine used for the materials has an effect of segregatingthe representative elements contained in the third region 103 on thesurface of the positive electrode active material 100 in a later step.

As the source of lithium, for example, lithium carbonate and lithiumfluoride can be used. As the source of the first transition metal, forexample, an oxide of the first transition metal can be used. As thesource of the representative element, for example, an oxide of therepresentative element contained in the third region and fluoride of therepresentative element contained in the third region can be used.

As the source of fluorine, for example, lithium fluoride and fluoride ofthe representative element contained in the third region can be used.That is, lithium fluoride can be used as either the source of lithium orthe source of fluorine.

The amount of fluorine contained in the source of fluorine is preferably1.0 time to 4 times (atomic ratio), further preferably 1.5 times to 3times (atomic ratio) the amount of representative element contained inthe source of the representative element.

<Step 12: Mixing of Starting Materials>

Next, the source of lithium, the source of the first transition metal,and the source of the representative element are mixed. In addition, thesource of fluorine is preferably added. For example, a ball mill and abead mill can be used for the mixing.

<Step 13: First Heating>

Next, the materials mixed in Step 12 are heated. In this step, theheating is referred to as baking or first heating in some cases. Theheating is preferably performed at higher than or equal to 800° C. andlower than or equal to 1100° C., further preferably at higher than orequal to 900° C. and lower than or equal to 1000° C. The heating time ispreferably greater than or equal to 2 hours and less than or equal to 20hours. The baking is preferably performed in a dried atmosphere such asdry air. In the dried atmosphere, for example, the dew point ispreferably lower than or equal to −50° C., further preferably lower thanor equal to −100° C. In this embodiment, the heating is performed at1000° C. for 10 hours, the temperature rising rate is 200° C./h, and dryair whose dew point is −109° C. flows at 10 L/min After that, the heatedmaterials are cooled to room temperature.

By the heating in Step 13, the composite oxide of lithium and the firsttransition metal having a layered rock-salt crystal structure can besynthesized. At this time, the representative element and fluorinecontained in the starting materials form a solid solution in thecomposite oxide. However, some representative elements have been alreadysegregated on the surface of the composite oxide in some cases.

In addition, as the starting materials, particles of the composite oxidecontaining lithium, cobalt, fluorine, and magnesium which aresynthesized in advance may be used. In this case, Step 12 and Step 13can be omitted. For example, lithium cobalt oxide particles (C-20F,produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.) can be used as one ofthe starting materials. The lithium cobalt oxide particle has a diameterof approximately 20 μm and contains fluorine, magnesium, calcium,sodium, silicon, sulfur, and phosphorus in a region which can beanalyzed by XPS from the surface.

<Step 14: Coating with Second Transition Metal>

Next, the composite oxide of lithium and the first transition metal iscooled to room temperature. Then, the surface of the composite oxideparticle of lithium and the first transition metal is coated with amaterial containing the second transition metal. In the formation methodexample, a sol-gel method is used.

First, the alkoxide of the second transition metal which is dissolved inalcohol and the composite oxide particles of lithium and the firsttransition metal are mixed.

For example, in the case where titanium is used as the second transitionmetal, TTIP can be used as the alkoxide of the second transition metal.As alcohol, isopropanol can be used, for example.

Next, the above mixed solution is stirred in an atmosphere containingwater vapor. The stirring can be performed with a magnetic stirrer, forexample. The stirring time is not limited as long as water and TTIP inan atmosphere cause hydrolysis and polycondensation reaction. Forexample, the stirring can be performed at 25° C. and a humidity of 90%RH (Relative Humidity) for 4 hours.

As described above, when water and TTIP in an atmosphere are reacted, asol-gel reaction can proceed more slowly as compared with the case whereliquid water is added. Alternatively, when titanium alkoxide and waterare reacted at room temperature, a sol-gel reaction can proceed moreslowly as compared with the case where heating is performed at atemperature higher than the boiling point of alcohol which is a solvent,for example. A sol-gel reaction proceeds slowly, whereby a high-qualitycoating layer containing titanium with a uniform thickness can beformed.

After the above process, precipitate is collected from the mixedsolution. As the collection method, filtration, centrifugation,evaporation and drying, or the like can be used. In this embodiment,filtration is used. For the filtration, a paper filter is used, and theresidue is washed by alcohol which is the same as the solvent in whichtitanium alkoxide is dissolved.

Then, the collected residue is dried. In this embodiment, vacuum dryingis performed at 70° C. for one hour.

<Step 15: Second Heating>

Next, the composite oxide particle coated with the material containingthe second transition metal, which is formed in Step 14, is heated. Thisstep is referred to as second heating in some cases. In the heating, theretention time within a specified temperature range is preferablyshorter than or equal to 50 hours, further preferably longer than orequal to 2 hours and shorter than or equal to 10 hours, still furtherpreferably longer than or equal to 1 hour and shorter than or equal to 3hours. If the heating time is too short, there is concern that thesegregation of the representative elements does not occur; however, ifthe heating time is too long, there is concern that the favorable secondregion 102 is not formed because diffusion of the second transitionmetal proceeds too much.

The specified temperature is preferably higher than or equal to 500° C.and lower than or equal to 1200° C., further preferably higher than orequal to 800° C. and lower than or equal to 1000° C. If the specifiedtemperature is too low, there is concern that the segregation of therepresentative elements and the second transition metal does not occur.However, if the specified temperature is too high, there is concern thatthe first transition metal in the composite oxide particle is reduced todecompose the composite oxide particle, that a layered structure oflithium and the first transition metal in the composite oxide particlecannot be kept, and the like.

In this embodiment, the specified temperature is 800° C. and kept for 2hours, the temperature rising rate is 200° C./h, and the flow rate ofdry air is 10 L/min.

By the heating in Step 15, the composite oxide of lithium and the firsttransition metal and the oxide of the second transition metal coveringthe composite oxide become topotaxy. In other words, the first region101 and the second region 102 become topotaxy.

By the heating in Step 15, the representative elements which form asolid solution inside the composite oxide particle of lithium and thefirst transition metal are unevenly distributed on the surface to form asolid solution, that is, the representative elements are segregated, thecompound of the representative elements is formed, and the third region103 is formed. At this time, the compound of the representative elementsis formed by heteroepitaxial growth from the second region 102. That is,the second region 102 and the third region 103 become topotaxy.

Since the second region 102 and the third region 103 contain crystalswhose orientations are substantially aligned with each other and have astable bond with the first region 101, when the positive electrodeactive material 100 is used for the secondary battery, a change of thecrystal structure in the first region 101 which is caused by chargingand discharging can be effectively suppressed. Even when lithium isreleased from the first region 101 due to charging, the superficialportion having a stable bond can suppress release of oxygen and thefirst transition metal such as cobalt from the first region 101.Furthermore, a chemically stable material can be used for a region incontact with the electrolyte solution. Thus, a secondary battery havingexcellent cycle characteristics can be provided.

Note that the entire first region 101 and second region 102 does notneed to become topotaxy as long as part of the first region 101 andsecond region 102 becomes topotaxy. Furthermore, the entire secondregion 102 and third region 103 does not need to become topotaxy as longas part of the second region 102 and third region 103 becomes topotaxy.

In the case where the compound of the representative elements containedin the third region contains oxygen, the heating in Step 15 ispreferably performed in an atmosphere containing oxygen. Heating in anatmosphere containing oxygen promotes the formation of the third region103.

Furthermore, fluorine contained in the starting materials promotes thesegregation of the representative elements.

In this manner, in the method for forming the positive electrode activematerial of one embodiment of the present invention, after the elementsforming the second region 102 are coated, heating is performed to formthe third region 103, and two kinds of regions can be formed on thesurface of the positive electrode active material 100. That is, ingeneral, two coating steps are necessary for providing two kinds ofregions in a superficial portion; however, in the method for forming thepositive electrode active material of one embodiment of the presentinvention, only one coating step (sol-gel process) is needed, which is aformation method with high productivity.

<Step 16: Cooling>

Next, the particles heated in Step 15 are cooled to room temperature.The time of decreasing temperature is preferably long because topotaxyis easily generated. For example, the time of decreasing temperaturefrom retention temperature to room temperature is preferably the same asthe time of increasing temperature or longer, specifically longer thanor equal to 10 hours and shorter than or equal to 50 hours.

<Step 17: Collecting>

Next, the cooled particles are collected. Moreover, the particles arepreferably made to pass through a sieve. Through the above process, thepositive electrode active material 100 including the first region 101,the second region 102, and the third region 103 can be formed.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 2

In this embodiment, examples of materials which can be used for asecondary battery containing the positive electrode active material 100described in the above embodiment are described. In this embodiment, asecondary battery in which a positive electrode, a negative electrode,and an electrolyte solution are wrapped in an exterior body is describedas an example

[Positive Electrode]

The positive electrode includes a positive electrode active materiallayer and a positive electrode current collector.

<Positive Electrode Active Material Layer>

The positive electrode active material layer contains at least apositive electrode active material. The positive electrode activematerial layer may contain, in addition to the positive electrode activematerial, other materials such as a coating film of the active materialsurface, a conductive additive, and a binder.

As the positive electrode active material, the positive electrode activematerial 100 described in the above embodiment can be used. When theabove-described positive electrode active material 100 is used, asecondary battery with high capacity and excellent cycle characteristicscan be obtained.

Examples of the conductive additive include a carbon material, a metalmaterial, and a conductive ceramic material. Alternatively, a fibermaterial may be used as the conductive additive. The content of theconductive additive with respect to the total amount of the activematerial layer is preferably greater than or equal to 1 wt % and lessthan or equal to 10 wt %, more preferably greater than or equal to 1 wt% and less than or equal to 5 wt %.

A network for electric conduction can be formed in the active materiallayer by the conductive additive. The conductive additive also allowsmaintaining of a path for electric conduction between the positiveelectrode active material particles. The addition of the conductiveadditive to the active material layer increases the electricconductivity of the active material layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, and fullerene. Alternatively, metal powder or metalfibers of copper, nickel, aluminum, silver, gold, or the like, aconductive ceramic material, or the like can be used.

Alternatively, a graphene compound may be used as the conductiveadditive.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength. Furthermore, a graphene compound has a planarshape. A graphene compound enables low-resistance surface contact.Furthermore, a graphene compound has extremely high conductivity evenwith a small thickness in some cases and thus allows a conductive pathto be formed in an active material layer efficiently even with a smallamount. For this reason, it is preferable to use a graphene compound asthe conductive additive because the area where the active material andthe conductive additive are in contact with each other can be increased.It is preferable to form the graphene compound serving as a conductiveadditive as a coating film to cover the entire surface of the activematerial with a spray dry apparatus, in which case the electricalresistance may be reduced. Here, it is particularly preferable to use,for example, graphene, multilayer graphene, or RGO as a graphenecompound. Note that RGO refers to a compound obtained by reducinggraphene oxide (GO), for example.

In the case where an active material with a small particle diameter(e.g., 1 μm or less) is used, the specific surface area of the activematerial is large and thus more conductive paths for the active materialparticles are needed. Thus, the amount of conductive additive tends toincrease and the supported amount of active material tends to decreaserelatively. When the supported amount of active material decreases, thecapacity of the secondary battery also decreases. In such a case, agraphene compound that can efficiently form a conductive path even in asmall amount is particularly preferably used as the conductive additivebecause the supported amount of active material does not decrease.

A cross-sectional structure example of an active material layer 200containing a graphene compound as a conductive additive is describedbelow.

FIG. 7A shows a longitudinal cross-sectional view of the active materiallayer 200. The active material layer 200 includes positive electrodeactive material particles 100, a graphene compound 201 serving as aconductive additive, and a binder (not illustrated). Here, graphene ormultilayer graphene may be used as the graphene compound 201, forexample. The graphene compound 201 preferably has a sheet-like shape.The graphene compound 201 may have a sheet-like shape formed of aplurality of sheets of multilayer graphene and/or a plurality of sheetsof graphene that partly overlap with each other.

The longitudinal cross section of the active material layer 200 in FIG.7B shows substantially uniform dispersion of the sheet-like graphenecompounds 201 in the active material layer 200. The graphene compounds201 are schematically shown by thick lines in FIG. 7B but are actuallythin films each having a thickness corresponding to the thickness of asingle layer or a multi-layer of carbon molecules. The plurality ofgraphene compounds 201 are formed in such a way as to partly coat oradhere to the surfaces of the plurality of positive electrode activematerial particles 100, so that the graphene compounds 201 make surfacecontact with the positive electrode active material particles 100.

Here, the plurality of graphene compounds are bonded to each other toform a net-like graphene compound sheet (hereinafter referred to as agraphene compound net or a graphene net). The graphene net covering theactive material can function as a binder for bonding active materials.The amount of a binder can thus be reduced, or the binder does not haveto be used. This can increase the proportion of the active material inthe electrode volume or weight. That is to say, the capacity of thesecondary battery can be increased.

Here, it is preferable to perform reduction after a layer to be theactive material layer 200 is formed in such a manner that graphene oxideis used as the graphene compound 201 and mixed with an active material.When graphene oxide with extremely high dispersibility in a polarsolvent is used for the formation of the graphene compounds 201, thegraphene compounds 201 can be substantially uniformly dispersed in theactive material layer 200. The solvent is removed by volatilization froma dispersion medium in which graphene oxide is uniformly dispersed, andthe graphene oxide is reduced; hence, the graphene compounds 201remaining in the active material layer 200 partly overlap with eachother and are dispersed such that surface contact is made, therebyforming a three-dimensional conduction path. Note that graphene oxidecan be reduced either by heat treatment or with the use of a reducingagent, for example.

Unlike a conductive additive in the form of particles, such as acetyleneblack, which makes point contact with an active material, the graphenecompound 201 is capable of making low-resistance surface contact;accordingly, the electrical conduction between the positive electrodeactive material particles 100 and the graphene compounds 201 can beimproved with a smaller amount of the graphene compound 201 than that ofa normal conductive additive. This increases the proportion of thepositive electrode active material particles 100 in the active materiallayer 200, resulting in increased discharge capacity of the secondarybattery.

Alternatively, the graphene compound may cover the entire surface of theactive material in advance with a spray dry apparatus. After that, atthe time of forming the positive electrode active material layer, agraphene compound can be further added to make the conductive pathbetween the active materials more favorable.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, or ethylene-propylene-diene copolymer can be used, forexample. Alternatively, fluororubber can be used as the binder.

For the binder, for example, water-soluble polymers are preferably used.As the water-soluble polymers, a polysaccharide and the like can beused. As the polysaccharide, a cellulose derivative such ascarboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,starch, or the like can be used. It is more preferred that suchwater-soluble polymers be used in combination with any of the aboverubber materials.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinylacetate, or nitrocellulose is preferably used.

A plurality of the above materials may be used in combination for thebinder.

For example, a material having a significant viscosity modifying effectand another material may be used in combination. For example, a rubbermaterial or the like has high adhesion or high elasticity but may havedifficulty in viscosity modification when mixed in a solvent. In such acase, a rubber material or the like is preferably mixed with a materialhaving a significant viscosity modifying effect, for example. As amaterial having a significant viscosity modifying effect, for example, awater-soluble polymer is preferably used. An example of a water-solublepolymer having an especially significant viscosity modifying effect isthe above-mentioned polysaccharide; for example, a cellulose derivativesuch as carboxymethyl cellulose (CMC), methyl cellulose, ethylcellulose, hydroxypropyl cellulose, diacetyl cellulose, or regeneratedcellulose, or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material and anothermaterial combined as a binder such as styrene-butadiene rubber in anaqueous solution. Furthermore, a water-soluble polymer is expected to beeasily and stably adsorbed to an active material surface because it hasa functional group. Many cellulose derivatives such as carboxymethylcellulose have functional groups such as a hydroxyl group and a carboxylgroup. Because of functional groups, polymers are expected to interactwith each other and cover an active material surface in a large area.

In the case where the binder covering or being in contact with theactive material surface forms a film, the film is expected to serve as apassivation film to suppress the decomposition of the electrolytesolution. Here, the passivation film refers to a film without electricconductivity or a film with extremely low electric conductivity, and caninhibit the decomposition of an electrolyte solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while suppressingelectric conduction.

<Positive Electrode Current Collector>

The positive electrode current collector can be formed using a materialthat has high conductivity, such as a metal like stainless steel, gold,platinum, aluminum, or titanium, or an alloy thereof. It is preferredthat a material used for the positive electrode current collector notdissolve at the potential of the positive electrode. Alternatively, thepositive electrode current collector can be formed using an aluminumalloy to which an element that improves heat resistance, such assilicon, titanium, neodymium, scandium, or molybdenum, is added. Stillalternatively, a metal element that forms silicide by reacting withsilicon can be used. Examples of the metal element that forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel.The current collector can have any of various shapes including afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a punching-metal shape, and an expanded-metal shape. The currentcollector preferably has a thickness of 5 μm to 30 μm.

[Negative Electrode]

The negative electrode includes a negative electrode active materiallayer and a negative electrode current collector. The negative electrodeactive material layer may contain a conductive additive and a binder.

<Negative Electrode Active Material>

As a negative electrode active material, for example, an alloy-basedmaterial or a carbon-based material can be used.

For the negative electrode active material, an element which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used. For example, a material containing atleast one of silicon, tin, gallium, aluminum, germanium, lead, antimony,bismuth, silver, zinc, cadmium, indium, and the like can be used. Suchelements have higher capacity than carbon. In particular, silicon has asignificantly high theoretical capacity of 4200 mAh/g. For this reason,silicon is preferably used as the negative electrode active material.Alternatively, a compound containing any of the above elements may beused. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn. Here, an element thatenables charge-discharge reactions by an alloying reaction and adealloying reaction with lithium, a compound containing the element, andthe like may be referred to as an alloy-based material.

In this specification and the like, SiO refers, for example, to siliconmonoxide. SiO can alternatively be expressed as SiO_(x). Here, xpreferably has an approximate value of 1. For example, x is preferably0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 orless.

As the carbon-based material, graphite, graphitizing carbon (softcarbon), non-graphitizing carbon (hard carbon), a carbon nanotube,graphene, carbon black, and the like can be used.

Examples of graphite include artificial graphite and natural graphite.Examples of artificial graphite include meso-carbon microbeads (MCMB),coke-based artificial graphite, and pitch-based artificial graphite. Asartificial graphite, spherical graphite having a spherical shape can beused. For example, MCMB is preferably used because it may have aspherical shape. Moreover, MCMB may preferably be used because it canrelatively easily have a small surface area. Examples of naturalgraphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.05 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferred because of its advantages such as arelatively high capacity per unit volume, relatively small volumeexpansion, low cost, and higher level of safety than that of a lithiummetal.

Alternatively, for the negative electrode active material, an oxide suchas titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active material,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active material; for example, a transitionmetal oxide which does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used.Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

For the conductive additive and the binder that can be included in thenegative electrode active material layer, materials similar to those ofthe conductive additive and the binder that can be included in thepositive electrode active material layer can be used.

<Negative Electrode Current Collector>

For the negative electrode current collector, a material similar to thatof the positive electrode current collector can be used. Note that amaterial which is not alloyed with a carrier ion such as lithium ispreferably used for the negative electrode current collector.

[Electrolyte Solution]

The electrolyte solution contains a solvent and an electrolyte. As asolvent of the electrolyte solution, an aprotic organic solvent ispreferably used. For example, one of ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate, chloroethylene carbonate, vinylenecarbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC),diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate,methyl acetate, ethyl acetate, methyl propionate, ethyl propionate,propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

Alternatively, when one or more kinds of ionic liquids (room temperaturemolten salts) which have features of non-flammability and non-volatilityis used as a solvent of the electrolyte solution, a secondary batterycan be prevented from exploding or catching fire even when the secondarybattery internally shorts out or the internal temperature increasesowing to overcharging or the like. An ionic liquid contains a cation andan anion. The ionic liquid contains an organic cation and an anion.Examples of the organic cation used for the electrolyte solution includealiphatic onium cations such as a quaternary ammonium cation, a tertiarysulfonium cation, and a quaternary phosphonium cation, and aromaticcations such as an imidazolium cation and a pyridinium cation. Examplesof the anion used for the electrolyte solution include a monovalentamide-based anion, a monovalent methide-based anion, a fluorosulfonateanion, a perfluoroalkylsulfonate anion, a tetrafluoroborate anion, aperfluoroalkylborate anion, a hexafluorophosphate anion, and aperfluoroalkylphosphate anion.

As an electrolyte dissolved in the above-described solvent, one oflithium salts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN,LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), andLiN(C₂F₅SO₂)₂ can be used, or two or more of these lithium salts can beused in an appropriate combination in an appropriate ratio.

The electrolyte solution used for a secondary battery is preferablyhighly purified and contains a small amount of dust particles andelements other than the constituent elements of the electrolyte solution(hereinafter also simply referred to as impurities). Specifically, theweight ratio of impurities to the electrolyte solution is less than orequal to 1%, preferably less than or equal to 0.1%, and furtherpreferably less than or equal to 0.01%.

Furthermore, an additive agent such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),LiBOB, or a dinitrile compound such as succinonitrile or adiponitrilemay be added to the electrolyte solution. The concentration of amaterial to be added with respect to the whole solvent is, for example,higher than or equal to 0.1 wt % and lower than or equal to 5 wt %.

Alternatively, a gelled electrolyte obtained in such a manner that apolymer is swelled with an electrolyte solution may be used.

When a polymer gel electrolyte is used, safety against liquid leakageand the like is improved. Furthermore, a secondary battery can bethinner and more lightweight.

As a polymer that undergoes gelation, a silicone gel, an acrylic gel, anacrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, a fluorine-based polymer gel, or the like can be used.

Examples of the polymer include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP) can be used. Theformed polymer may be porous.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including ahigh-molecular material such as a polyethylene oxide (PEO)-basedhigh-molecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, since the battery can be entirely solidified, there is nopossibility of liquid leakage to increase the safety of the batterydramatically.

[Separator]

The secondary battery preferably includes a separator. As the separator,for example, paper; nonwoven fabric; glass fiber; ceramics; or syntheticfiber using nylon (polyamide), vinylon (polyvinyl alcohol-based fiber),polyester, acrylic, polyolefin, or polyurethane can be used. Theseparator is preferably formed to have an envelope-like shape to wrapone of the positive electrode and the negative electrode.

The separator may have a multilayer structure. For example, an organicmaterial film such as polypropylene or polyethylene can be coated with aceramic-based material, a fluorine-based material, a polyamide-basedmaterial, a mixture thereof, or the like. Examples of the ceramic-basedmaterial include aluminum oxide particles and silicon oxide particles.Examples of the fluorine-based material include PVDF and apolytetrafluoroethylene. Examples of the polyamide-based materialinclude nylon and aramid (meta-based aramid and para-based aramid).

Deterioration of the separator in charging and discharging at highvoltage can be suppressed and thus the reliability of the secondarybattery can be improved because oxidation resistance is improved whenthe separator is coated with the ceramic-based material. In addition,when the separator is coated with the fluorine-based material, theseparator is easily brought into close contact with an electrode,resulting in high output characteristics. When the separator is coatedwith the polyamide-based material, in particular, aramid, the safety ofthe secondary battery is improved because heat resistance is improved.

For example, both surfaces of a polypropylene film may be coated with amixed material of aluminum oxide and aramid. Alternatively, a surface ofthe polypropylene film in contact with the positive electrode may becoated with the mixed material of aluminum oxide and aramid, and asurface of the polypropylene film in contact with the negative electrodemay be coated with the fluorine-based material.

With the use of a separator having a multilayer structure, the capacityof the secondary battery per volume can be increased because the safetyof the secondary battery can be maintained even when the total thicknessof the separator is small.

[Exterior Body]

For an exterior body included in the secondary battery, a metal materialsuch as aluminum and a resin material can be used, for example. Anexterior body in the form of a film can also be used. As the film, forexample, a film having a three-layer structure in which a highlyflexible metal thin film of aluminum, stainless steel, copper, nickel,or the like is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, or polyamide, andan insulating synthetic resin film of a polyamide-based resin, apolyester-based resin, or the like is provided as the outer surface ofthe exterior body over the metal thin film can be used.

[Charging and Discharging Methods]

The secondary battery can be charged and discharged in the followingmanner, for example.

<<CC Charge>>

First, CC charge, which is one of charging methods, is described. CCcharge is a charging method in which a constant current is made to flowto a secondary battery in the whole charging period and charge isterminated when the voltage reaches a predetermined voltage. Thesecondary battery is assumed to be an equivalent circuit with internalresistance R and secondary battery capacitance C as illustrated in FIG.8A. In that case, a secondary battery voltage V_(B) is the sum of avoltage V_(R) applied to the internal resistance R and a voltage V_(C)applied to the secondary battery capacitance C.

While the CC charge is performed, a switch is on as illustrated in FIG.8A, so that a constant current I flows to the secondary battery. Duringthe period, the current I is constant; thus, according to the Ohm's law(V_(R)=R×I), the voltage V_(R) applied to the internal resistance R isalso constant. In contrast, the voltage V_(C) applied to the secondarybattery capacitance C increases over time. Accordingly, the secondarybattery voltage V_(B) increases over time.

When the secondary battery voltage V_(B) reaches a predeterminedvoltage, e.g., 4.3 V, the charge is terminated. On termination of the CCcharge, the switch is turned off as illustrated in FIG. 8B, and thecurrent I becomes 0. Thus, the voltage V_(R) applied to the internalresistance R becomes 0 V. Consequently, the secondary battery voltageV_(B) is decreased by the lost voltage drop in the internal resistanceR.

FIG. 8C shows an example of the secondary battery voltage V_(B) andcharging current during a period in which the CC charge is performed andafter the CC charge is terminated. The secondary battery voltage V_(B)increases while the CC charge is performed, and slightly decreases afterthe CC charge is terminated.

<<CCCV Charge>>

Next, CCCV charge, which is a charging method different from theabove-described method, is described. CCCV charge is a charging methodin which CC charge is performed until the voltage reaches apredetermined voltage and then constant voltage (CV) charge is performeduntil the amount of current flow becomes small, specifically, atermination current value.

While the CC charge is performed, a switch of a constant current powersource is on and a switch of a constant voltage power source is off asillustrated in FIG. 9A, so that the constant current I flows to thesecondary battery. During the period, the current I is constant; thus,according to the Ohm's law (V_(R)=R×I), the voltage V_(R) applied to theinternal resistance R is also constant. In contrast, the voltage V_(C)applied to the secondary battery capacitance C increases over time.Accordingly, the secondary battery voltage V_(B) increases over time.

When the secondary battery voltage V_(B) reaches a predeterminedvoltage, e.g., 4.3 V, switching is performed from the CC charge to theCV charge. While the CV charge is performed, the switch of the constantvoltage power source is on and the switch of the constant current powersource is off as illustrated in FIG. 9B; thus, the secondary batteryvoltage V_(B) is constant. In contrast, the voltage V_(C) applied to thesecondary battery capacitance C increases over time. SinceV_(B)=V_(R)+V_(C) is satisfied, the voltage V_(R) applied to theinternal resistance R decreases over time. As the voltage V_(R) appliedto the internal resistance R decreases, the current I flowing to thesecondary battery also decreases according to the Ohm's law (V_(R)=R×I).

When the current I flowing to the secondary battery becomes apredetermined current, e.g., approximately 0.01 C, charge is terminated.On termination of the CCCV charge, all the switches are turned off asillustrated in FIG. 9C, so that the current I becomes 0. Thus, thevoltage V_(R) applied to the internal resistance R becomes 0 V. However,the voltage V_(R) applied to the internal resistance R becomessufficiently small by the CV charge; thus, even when a voltage drop nolonger occurs in the internal resistance R, the secondary batteryvoltage V_(B) hardly decreases.

FIG. 9D shows an example of the secondary battery voltage V_(B) andcharging current while the CCCV charge is performed and after the CCCVcharge is terminated. Even after the CCCV charge is terminated, thesecondary battery voltage V_(B) hardly decreases.

<<CC Discharge>>

Next, CC discharge, which is one of discharging methods, is described.CC discharge is a discharging method in which a constant current is madeto flow from the secondary battery in the whole discharging period, anddischarge is terminated when the secondary battery voltage V_(B) reachesa predetermined voltage, e.g., 2.5 V.

FIG. 10 shows an example of the secondary battery voltage V_(B) anddischarging current while the CC discharge is performed. As dischargeproceeds, the secondary battery voltage V_(B) decreases.

Next, a discharge rate and a charge rate will be described. Thedischarge rate refers to the relative ratio of discharging current tobattery capacity and is expressed in a unit C. A current ofapproximately 1 C in a battery with a rated capacity X (Ah) is X A. Thecase where discharge is performed at a current of 2X A is rephrased asfollows: discharge is performed at 2 C. The case where discharge isperformed at a current of X/5 A is rephrased as follows: discharge isperformed at 0.2 C. Similarly, the case where charging is performed at acurrent of 2X A is rephrased as follows: charging is performed at 2 C,and the case where charging is performed at a current of X/5 A isrephrased as follows: charging is performed at 0.2 C.

Embodiment 3

In this embodiment, examples of a shape of a secondary batterycontaining the positive electrode active material 100 described in theabove embodiment are described. For the materials used for the secondarybattery described in this embodiment, the description of the aboveembodiment can be referred to.

[Coin-Type Secondary Battery]

First, an example of a coin-type secondary battery is described. FIG.11A is an external view of a coin-type (single-layer flat type)secondary battery, and FIG. 11B is a cross-sectional view thereof.

In a coin-type secondary battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type secondary battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolyte solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel) can beused. Alternatively, the positive electrode can 301 and the negativeelectrode can 302 are preferably covered with nickel, aluminum, or thelike in order to prevent corrosion due to the electrolyte solution. Thepositive electrode can 301 and the negative electrode can 302 areelectrically connected to the positive electrode 304 and the negativeelectrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte solution. Then, asillustrated in FIG. 11B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303located therebetween. In such a manner, the coin-type secondary battery300 can be manufactured.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 304, the coin-typesecondary battery 300 with high capacity and excellent cyclecharacteristics can be obtained.

Here, a current flow in charging a secondary battery is described withreference to FIG. 11C. When a secondary battery using lithium isregarded as a closed circuit, lithium ions transfer and a current flowsin the same direction. Note that in the secondary battery using lithium,an anode and a cathode change places in charge and discharge, and anoxidation reaction and a reduction reaction occur on the correspondingsides; hence, an electrode with a high reaction potential is called apositive electrode and an electrode with a low reaction potential iscalled a negative electrode. For this reason, in this specification, thepositive electrode is referred to as a “positive electrode” or a “pluselectrode” and the negative electrode is referred to as a “negativeelectrode” or a “minus electrode” in all the cases where charge isperformed, discharge is performed, a reverse pulse current is supplied,and a charging current is supplied. The use of the terms “anode” and“cathode” related to an oxidation reaction and a reduction reactionmight cause confusion because the anode and the cathode change places atthe time of charging and discharging. Thus, the terms “anode” and“cathode” are not used in this specification. If the term “anode” or“cathode” is used, it should be mentioned that the anode or the cathodeis which of the one at the time of charging or the one at the time ofdischarging and corresponds to which of a positive (plus) electrode or anegative (minus) electrode.

Two terminals in FIG. 11C are connected to a charger, and the coin-typesecondary battery 300 is charged. As the charge of the coin-typesecondary battery 300 proceeds, a potential difference betweenelectrodes increases.

[Cylindrical Secondary Battery]

Next, an example of a cylindrical secondary battery will be describedwith reference to FIGS. 12A to 12D. A cylindrical secondary battery 600includes, as illustrated in FIG. 12A, a positive electrode cap (batterylid) 601 on the top surface and a battery can (outer can) 602 on theside and bottom surfaces. The positive electrode cap and the battery can(outer can) 602 are insulated from each other by a gasket (insulatingpacking) 610.

FIG. 12B is a schematic cross-sectional view of the cylindricalsecondary battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astrip-like separator 605 located therebetween is provided. Although notillustrated, the battery element is wound around a center pin. One endof the battery can 602 is close and the other end thereof is open. Forthe battery can 602, a metal having a corrosion-resistant property to anelectrolyte solution, such as nickel, aluminum, or titanium, an alloy ofsuch a metal, or an alloy of such a metal and another metal (e.g.,stainless steel) can be used. Alternatively, the battery can 602 ispreferably covered with nickel, aluminum, or the like in order toprevent corrosion due to the electrolyte solution. Inside the batterycan 602, the battery element in which the positive electrode, thenegative electrode, and the separator are wound is provided between apair of insulating plates 608 and 609 that face each other. Furthermore,a nonaqueous electrolyte solution (not illustrated) is injected insidethe battery can 602 provided with the battery element. As the nonaqueouselectrolyte solution, a nonaqueous electrolyte solution that is similarto that of the coin-type secondary battery can be used.

Since the positive electrode and the negative electrode of thecylindrical secondary battery are wound, active materials are preferablyformed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. The PTC element 611, which serves as a thermally sensitiveresistor whose resistance increases as temperature rises, limits theamount of current by increasing the resistance, in order to preventabnormal heat generation. Barium titanate (BaTiO₃)-based semiconductorceramic can be used for the PTC element.

Alternatively, as illustrated in FIG. 12C, a plurality of cylindricalsecondary batteries 600 may be sandwiched between a conductive plate 613and a conductive plate 614 to form a module 615. The plurality ofcylindrical secondary batteries 600 may be connected in parallel,connected in series, or connected in series after being connected inparallel. With the module 615 including the plurality of cylindricalsecondary batteries 600, large electric power can be extracted.

FIG. 12D is a top view of the module 615. The conductive plate 613 isshown by a dotted line for clarity of the drawing. As illustrated inFIG. 12D, the module 615 may include a wiring 616 which electricallyconnects the plurality of cylindrical secondary batteries 600 to eachother. It is possible to provide the conductive plate 613 over thewiring 616 to overlap with each other. In addition, a temperaturecontrol device 617 may be provided between the plurality of cylindricalsecondary batteries 600. When the cylindrical secondary batteries 600are overheated, the temperature control device 617 can cool them, andwhen the cylindrical secondary batteries 600 are cooled too much, thetemperature control device 617 can heat them. Thus, the performance ofthe module 615 is not easily influenced by the outside air temperature.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 604, the cylindricalsecondary battery 600 with high capacity and excellent cyclecharacteristics can be obtained.

[Structural Examples of Secondary Battery]

Other structural examples of secondary batteries will be described withreference to FIGS. 13A and 13B, FIGS. 14A-1, 14A-2, 14B-1, and 14B-2,FIGS. 15A and 15B, and FIG. 16.

FIGS. 13A and 13B are external views of a secondary battery. Thesecondary battery includes a circuit board 900 and a secondary battery913. A label 910 is attached to the secondary battery 913. As shown inFIG. 13B, the secondary battery further includes a terminal 951, aterminal 952, an antenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. The shape of each of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape. Furthermore, aplanar antenna, an aperture antenna, a traveling-wave antenna, an EHantenna, a magnetic-field antenna, a dielectric antenna, or the like maybe used. Alternatively, the antenna 914 or the antenna 915 may be aflat-plate conductor. The flat-plate conductor can serve as one ofconductors for electric field coupling. That is, the antenna 914 or theantenna 915 can serve as one of two conductors of a capacitor. Thus,electric power can be transmitted and received not only by anelectromagnetic field or a magnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The secondary battery includes a layer 916 between the secondary battery913 and the antennas 914 and 915. The layer 916 has a function ofblocking an electromagnetic field from the secondary battery 913, forexample. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the secondary battery is not limited to thatshown in FIGS. 13A and 13B.

For example, as shown in FIGS. 14A-1 and 14A-2, two opposite surfaces ofthe secondary battery 913 in FIGS. 13A and 13B may be provided withrespective antennas. FIG. 14A-1 is an external view showing one side ofthe opposite surfaces, and FIG. 14A-2 is an external view showing theother side of the opposite surfaces. For portions similar to those inFIGS. 13A and 13B, a description of the secondary battery illustrated inFIGS. 13A and 13B can be referred to as appropriate.

As illustrated in FIG. 14A-1, the antenna 914 is provided on one of theopposite surfaces of the secondary battery 913 with the layer 916located therebetween, and as illustrated in FIG. 14A-2, an antenna 918is provided on the other of the opposite surfaces of the secondarybattery 913 with a layer 917 located therebetween. The layer 917 has afunction of blocking an electromagnetic field from the secondary battery913, for example. As the layer 917, for example, a magnetic body can beused.

With the above structure, both of the antennas 914 and 918 can beincreased in size. The antenna 918 has a function of communicating datawith an external device, for example. An antenna with a shape that canbe applied to the antenna 914, for example, can be used as the antenna918. As a system for communication using the antenna 918 between thesecondary battery and another device, a response method that can be usedbetween the secondary battery and another device, such as NFC, can beemployed.

Alternatively, as illustrated in FIG. 14B-1, the secondary battery 913in FIGS. 13A and 13B may be provided with a display device 920. Thedisplay device 920 is electrically connected to the terminal 911. It ispossible that the label 910 is not provided in a portion where thedisplay device 920 is provided. For portions similar to those in FIGS.13A and 13B, a description of the secondary battery illustrated in FIGS.13A and 13B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charge is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 14B-2, the secondary battery 913illustrated in FIGS. 13A and 13B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 13A and 13B, a descriptionof the secondary battery illustrated in FIGS. 13A and 13B can bereferred to as appropriate.

The sensor 921 has a function of measuring, for example, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, electric current, voltage,electric power, radiation, flow rate, humidity, gradient, oscillation,odor, or infrared rays. With the sensor 921, for example, data on anenvironment (e.g., temperature) where the secondary battery is placedcan be determined and stored in a memory inside the circuit 912.

Furthermore, structural examples of the secondary battery 913 will bedescribed with reference to FIGS. 15A and 15B and FIG. 16.

The secondary battery 913 illustrated in FIG. 15A includes a wound body950 provided with the terminals 951 and 952 inside a housing 930. Thewound body 950 is soaked in an electrolyte solution inside the housing930. The terminal 952 is in contact with the housing 930. An insulatoror the like inhibits contact between the terminal 951 and the housing930. Note that in FIG. 15A, the housing 930 divided into two pieces isillustrated for convenience; however, in the actual structure, the woundbody 950 is covered with the housing 930 and the terminals 951 and 952extend to the outside of the housing 930. For the housing 930, a metalmaterial (such as aluminum) or a resin material can be used.

Note that as illustrated in FIG. 15B, the housing 930 in FIG. 15A may beformed using a plurality of materials. For example, in the secondarybattery 913 in FIG. 15B, a housing 930 a and a housing 930 b are bondedto each other, and the wound body 950 is provided in a region surroundedby the housing 930 a and the housing 930 b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, blocking of an electricfield from the secondary battery 913 can be inhibited. When an electricfield is not significantly blocked by the housing 930 a, an antenna suchas the antennas 914 and 915 may be provided inside the housing 930 a.For the housing 930 b, a metal material can be used, for example.

FIG. 16 illustrates the structure of the wound body 950. The wound body950 includes a negative electrode 931, a positive electrode 932, andseparators 933. The wound body 950 is obtained by winding a sheet of astack in which the negative electrode 931 overlaps with the positiveelectrode 932 with the separator 933 provided therebetween. Note that aplurality of stacks each including the negative electrode 931, thepositive electrode 932, and the separator 933 may be stacked.

The negative electrode 931 is connected to the terminal 911 in FIGS. 13Aand 13B via one of the terminals 951 and 952. The positive electrode 932is connected to the terminal 911 in FIGS. 13A and 13B via the other ofthe terminals 951 and 952.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 932, the secondary battery913 with high capacity and excellent cycle characteristics can beobtained.

[Laminated Secondary Battery]

Next, an example of a laminated secondary battery will be described withreference to FIGS. 17A to 17C, FIGS. 18A and 18B, FIG. 19, FIG. 20,FIGS. 21A to 21C, FIGS. 22A, 22B1, 22B2, 22C, and 22D, and FIGS. 23A and23B. When the laminated secondary battery has flexibility and is used inan electronic device at least part of which is flexible, the secondarybattery can be bent as the electronic device is bent.

A laminated secondary battery 980 is described with reference to FIGS.17A to 17C. The laminated secondary battery 980 includes a wound body993 illustrated in FIG. 17A. The wound body 993 includes a negativeelectrode 994, a positive electrode 995, and a separator 996. The woundbody 993 is, like the wound body 950 illustrated in FIG. 16, obtained bywinding a sheet of a stack in which the negative electrode 994 overlapswith the positive electrode 995 with the separator 996 therebetween.

Note that the number of stacks each including the negative electrode994, the positive electrode 995, and the separator 996 may be determinedas appropriate depending on capacity and an element volume which arerequired. The negative electrode 994 is connected to a negativeelectrode current collector (not illustrated) via one of a leadelectrode 997 and a lead electrode 998. The positive electrode 995 isconnected to a positive electrode current collector (not illustrated)via the other of the lead electrode 997 and the lead electrode 998.

As illustrated in FIG. 17B, the wound body 993 is packed in a spaceformed by bonding a film 981 and a film 982 having a depressed portionthat serve as exterior bodies by thermocompression bonding or the like,whereby the laminated secondary battery 980 can be formed as illustratedin FIG. 17C. The wound body 993 includes the lead electrode 997 and thelead electrode 998, and is soaked in an electrolyte solution inside aspace surrounded by the film 981 and the film 982 having a depressedportion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for exampleWith the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be changed in their forms when external force isapplied; thus, a flexible secondary battery can be fabricated.

Although FIGS. 17B and 17C illustrate an example where a space is formedby two films, the wound body 993 may be placed in a space formed bybending one film.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 995, the laminatedsecondary battery 980 with high capacity and excellent cyclecharacteristics can be obtained.

In FIGS. 17A to 17C, an example in which the laminated secondary battery980 includes a wound body in a space formed by films serving as exteriorbodies is described; however, as illustrated in FIGS. 18A and 18B, asecondary battery may include a plurality of strip-shaped positiveelectrodes, a plurality of strip-shaped separators, and a plurality ofstrip-shaped negative electrodes in a space formed by films serving asexterior bodies, for example.

A laminated secondary battery 500 illustrated in FIG. 18A includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolyte solution 508, and an exterior body 509. The separator 507 isprovided between the positive electrode 503 and the negative electrode506 in the exterior body 509. The exterior body 509 is filled with theelectrolyte solution 508. The electrolyte solution described inEmbodiment 2 can be used for the electrolyte solution 508.

In the laminated secondary battery 500 illustrated in FIG. 18A, thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also serve as terminals for an electrical contactwith an external portion. For this reason, the positive electrodecurrent collector 501 and the negative electrode current collector 504may be arranged so as to be partly exposed to the outside of theexterior body 509. Alternatively, a lead electrode and the positiveelectrode current collector 501 or the negative electrode currentcollector 504 may be bonded to each other by ultrasonic welding, andinstead of the positive electrode current collector 501 and the negativeelectrode current collector 504, the lead electrode may be exposed tothe outside of the exterior body 509.

As the exterior body 509 of the laminated secondary battery 500, forexample, a laminate film having a three-layer structure can be employedin which a highly flexible metal thin film of aluminum, stainless steel,copper, nickel, or the like is provided over a film formed of a materialsuch as polyethylene, polypropylene, polycarbonate, ionomer, orpolyamide, and an insulating synthetic resin film of a polyamide-basedresin, a polyester-based resin, or the like is provided over the metalthin film as the outer surface of the exterior body.

FIG. 18B illustrates an example of a cross-sectional structure of thelaminated secondary battery 500. Although FIG. 18A illustrates anexample including only two current collectors for simplicity, an actualbattery includes a plurality of electrode layers.

The example in FIG. 18B includes 16 electrode layers. The laminatedsecondary battery 500 has flexibility even though including 16 electrodelayers. FIG. 18B illustrates a structure including 8 layers of negativeelectrode current collectors 504 and 8 layers of positive electrodecurrent collectors 501, i.e., 16 layers in total. Note that FIG. 18Billustrates a cross section of the lead portion of the negativeelectrode, and the 8 negative electrode current collectors 504 arebonded to each other by ultrasonic welding. It is needless to say thatthe number of electrode layers is not limited to 16, and may be morethan 16 or less than 16. With a large number of electrode layers, thesecondary battery can have high capacity. In contrast, with a smallnumber of electrode layers, the secondary battery can have smallthickness and high flexibility.

FIGS. 19 and 20 each illustrate an example of the external view of thelaminated secondary battery 500. In FIGS. 19 and 20, the positiveelectrode 503, the negative electrode 506, the separator 507, theexterior body 509, a positive electrode lead electrode 510, and anegative electrode lead electrode 511 are included.

FIG. 21A illustrates external views of the positive electrode 503 andthe negative electrode 506. The positive electrode 503 includes thepositive electrode current collector 501, and the positive electrodeactive material layer 502 is formed on a surface of the positiveelectrode current collector 501. The positive electrode 503 alsoincludes a region where the positive electrode current collector 501 ispartly exposed (hereinafter referred to as a tab region). The negativeelectrode 506 includes the negative electrode current collector 504, andthe negative electrode active material layer 505 is formed on a surfaceof the negative electrode current collector 504. The negative electrode506 also includes a region where the negative electrode currentcollector 504 is partly exposed, that is, a tab region. The areas andthe shapes of the tab regions included in the positive electrode and thenegative electrode are not limited to those illustrated in FIG. 21A.

[Method for Manufacturing Laminated Secondary Battery]

Here, an example of a method for manufacturing the laminated secondarybattery whose external view is illustrated in FIG. 19 will be describedwith reference to FIGS. 21B and 21C.

First, the negative electrode 506, the separator 507, and the positiveelectrode 503 are stacked. FIG. 21B illustrates a stack including thenegative electrode 506, the separator 507, and the positive electrode503. The secondary battery described here as an example includes 5negative electrodes and 4 positive electrodes. Next, the tab regions ofthe positive electrodes 503 are bonded to each other, and the tab regionof the positive electrode on the outermost surface and the positiveelectrode lead electrode 510 are bonded to each other. The bonding canbe performed by ultrasonic welding, for example. In a similar manner,the tab regions of the negative electrodes 506 are bonded to each other,and the negative electrode lead electrode 511 is bonded to the tabregion of the negative electrode on the outermost surface.

After that, the negative electrode 506, the separator 507, and thepositive electrode 503 are placed over the exterior body 509.

Subsequently, the exterior body 509 is folded along a dashed line asillustrated in FIG. 21C. Then, the outer edge of the exterior body 509is bonded. The bonding can be performed by thermocompression bonding,for example. At this time, a part (or one side) of the exterior body 509is left unbonded (to provide an inlet) so that the electrolyte solution508 can be introduced later.

Next, the electrolyte solution 508 is introduced into the exterior body509 from the inlet of the exterior body 509. The electrolyte solution508 is preferably introduced in a reduced pressure atmosphere or in aninert gas atmosphere. Lastly, the inlet is bonded. In the above manner,the laminated secondary battery 500 can be manufactured.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 503, the laminatedsecondary battery 500 with high capacity and excellent cyclecharacteristics can be obtained.

[Bendable Secondary Battery]

Next, an example of a bendable secondary battery is described withreference to FIGS. 22A, 22B1, 22B2, 22C and 22D and FIGS. 23A and 23B.

FIG. 22A is a schematic top view of a bendable secondary battery 50.FIGS. 22B1, 22B2, and 22C are schematic cross-sectional views takenalong cutting line C1-C2, cutting line C3-C4, and cutting line A1-A2,respectively, in FIG. 22A. The battery 50 includes an exterior body 51and a positive electrode 11 a, and a negative electrode 11 b held in theexterior body 51. A lead 12 a electrically connected to the positiveelectrode 11 a and a lead 12 b electrically connected to the negativeelectrode 11 b are extended to the outside of the exterior body 51. Inaddition to the positive electrode 11 a and the negative electrode 11 b,an electrolyte solution (not illustrated) is enclosed in a regionsurrounded by the exterior body 51.

FIGS. 23A and 23B illustrate the positive electrode 11 a and thenegative electrode 11 b included in the battery 50. FIG. 23A is aperspective view illustrating the stacking order of the positiveelectrode 11 a, the negative electrode 11 b, and the separator 14. FIG.23B is a perspective view illustrating the lead 12 a and the lead 12 bin addition to the positive electrode 11 a and the negative electrode 11b.

As illustrated in FIG. 23A, the battery 50 includes a plurality ofstrip-shaped positive electrodes 11 a, a plurality of strip-shapednegative electrodes 11 b, and a plurality of separators 14. The positiveelectrode 11 a and the negative electrode 11 b each include a projectedtab portion and a portion other than the tab. A positive electrodeactive material layer is formed on one surface of the positive electrode11 a other than the tab portion, and a negative electrode activematerial layer is formed on one surface of the negative electrode 11 bother than the tab portion.

The positive electrodes 11 a and the negative electrodes 11 b arestacked so that surfaces of the positive electrodes 11 a on each ofwhich the positive electrode active material layer is not formed are incontact with each other and that surfaces of the negative electrodes 11b on each of which the negative electrode active material layer is notformed are in contact with each other.

Furthermore, the separator 14 is provided between the surface of thepositive electrode 11 a on which the positive electrode active materialis formed and the surface of the negative electrode 11 b on which thenegative electrode active material is formed. In FIG. 23A, the separator14 is shown by a dotted line for easy viewing.

In addition, as illustrated in FIG. 23B, the plurality of positiveelectrodes 11 a are electrically connected to the lead 12 a in a bondingportion 15 a. The plurality of negative electrodes 11 b are electricallyconnected to the lead 12 b in a bonding portion 15 b.

Next, the exterior body 51 is described with reference to FIGS. 22B1,22B2, 22C, and 22D.

The exterior body 51 has a film-like shape and is folded in half withthe positive electrodes 11 a and the negative electrodes 11 b betweenfacing portions of the exterior body 51. The exterior body 51 includes afolded portion 61, a pair of seal portions 62, and a seal portion 63.The pair of seal portions 62 is provided with the positive electrodes 11a and the negative electrodes 11 b positioned therebetween and thus canalso be referred to as side seals. The seal portion 63 has portionsoverlapping with the lead 12 a and the lead 12 b and can also bereferred to as a top seal.

Part of the exterior body 51 that overlaps with the positive electrodes11 a and the negative electrodes 11 b preferably has a wave shape inwhich crest lines 71 and trough lines 72 are alternately arranged. Theseal portions 62 and the seal portion 63 of the exterior body 51 arepreferably flat.

FIG. 22B1 shows a cross section cut along the part overlapping with thecrest line 71. FIG. 22B2 shows a cross section cut along the partoverlapping with the trough line 72. FIGS. 22B1 and 22B2 correspond tocross sections of the battery 50, the positive electrodes 11 a, and thenegative electrodes 11 b in the width direction.

The distance between an end portion of the negative electrode 11 b inthe width direction and the seal portion 62 is referred to as a distanceLa. When the battery 50 changes in shape, for example, is bent, thepositive electrode 11 a and the negative electrode 11 b change in shapesuch that the positions thereof are shifted from each other in thelength direction as described later. At the time, if the distance La istoo short, the exterior body 51 and the positive electrode 11 a and thenegative electrode 11 b are rubbed hard against each other, so that theexterior body 51 is damaged in some cases. In particular, when a metalfilm of the exterior body 51 is exposed, there is concern that the metalfilm is corroded by the electrolyte solution. Thus, the distance La ispreferably set as long as possible. However, if the distance La is toolong, the volume of the battery 50 is increased.

The distance La between the end portion of the negative electrode 11 band the seal portion 62 is preferably increased as the total thicknessof the stacked positive electrodes 11 a and negative electrodes 11 b isincreased.

Specifically, when the total thickness of the stacked positiveelectrodes 11 a and negative electrodes 11 b and the separators 214 isreferred to as a thickness t, the distance La is preferably 0.8 times ormore and 3.0 times or less, further preferably 0.9 times or more and 2.5times or less, still further preferably 1.0 times or more and 2.0 timesor less as large as the thickness t. When the distance La is in theabove-described range, a compact battery which is highly reliable forbending can be obtained.

Furthermore, when a distance between the pair of seal portions 62 isreferred to as a distance Lb, it is preferable that the distance Lb besufficiently longer than a width Wb of the negative electrode 11 b. Inthis case, even when the positive electrode 11 a and the negativeelectrode 11 b come into contact with the exterior body 51 by change inthe shape of the battery 50 such as repeated bending, the position ofpart of the positive electrode 11 a and the negative electrode 11 b canbe shifted in the width direction; thus, the positive and negativeelectrodes 11 a and 11 b and the exterior body 51 can be effectivelyprevented from being rubbed against each other.

For example, the difference between the distance Lb (i.e., the distancebetween the pair of seal portions 62) and the width Wb of the negativeelectrode 11 b is preferably 1.6 times or more and 6.0 times or less,further preferably 1.8 times or more and 5.0 times or less, stillfurther preferably 2.0 times or more and 4.0 times or less as large asthe total thickness t of the positive electrode 11 a and the negativeelectrode 11 b.

In other words, the distance Lb, the width Wb, and the thickness tpreferably satisfy the relation of the following Formula 2.

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\mspace{526mu}} & \; \\{\frac{{Lb} - {Wb}}{2t} \geq a} & \left( {{Formula}\mspace{14mu} 2} \right)\end{matrix}$

In the formula, a is 0.8 or more and 3.0 or less, preferably 0.9 or moreand 2.5 or less, further preferably 1.0 or more and 2.0 or less.

FIG. 22C illustrates a cross section including the lead 12 a andcorresponds to a cross section of the battery 50, the positive electrode11 a, and the negative electrode 11 b in the length direction. Asillustrated in FIG. 22C, a space 73 is preferably provided between endportions of the positive electrode 11 a and the negative electrode 11 bin the length direction and the exterior body 51 in the folded portion61.

FIG. 22D is a schematic cross-sectional view of the battery 50 in astate of being bent. FIG. 22D corresponds to a cross section alongcutting line B1-B2 in FIG. 22A.

When the battery 50 is bent, a part of the exterior body 51 positionedon the outer side in bending is unbent and the other part positioned onthe inner side changes its shape as it shrinks. More specifically, thepart of the exterior body 51 positioned on the outer side in bendingchanges its shape such that the wave amplitude becomes smaller and thelength of the wave period becomes larger. In contrast, the part of theexterior body 51 positioned on the inner side in bending changes itsshape such that the wave amplitude becomes larger and the length of thewave period becomes smaller. When the exterior body 51 changes its shapein this manner, stress applied to the exterior body 51 due to bending isrelieved, so that a material itself that forms the exterior body 51 doesnot need to expand and contract. As a result, the battery 50 can be bentwith weak force without damage to the exterior body 51.

Furthermore, as illustrated in FIG. 22D, when the battery 50 is bent,the positions of the positive electrode 11 a and the negative electrode11 b are shifted relatively. At this time, ends of the stacked positiveelectrodes 11 a and negative electrodes 11 b on the seal portion 63 sideare fixed by the fixing member 17. Thus, the plurality of positiveelectrodes 11 a and the plurality of negative electrodes 11 b are moreshifted at a position closer to the folded portion 61. Therefore, stressapplied to the positive electrode 11 a and the negative electrode 11 bis relieved, and the positive electrode 11 a and the negative electrode11 b themselves do not need to expand and contract. As a result, thebattery 50 can be bent without damage to the positive electrode 11 a andthe negative electrode 11 b.

Furthermore, the space 73 is provided between the end portions of thepositive and negative electrodes 11 a and 11 b and the exterior body 51,whereby the relative positions of the positive electrode 11 a and thenegative electrode 11 b can be shifted while the end portions of thepositive electrode 11 a and the negative electrode 11 b located on aninner side when the battery 50 is bent do not contact the exterior body51.

In the battery 50 illustrated in FIGS. 22A, 22B1, 22B2, 22C and 22D andFIGS. 23A and 23B, the exterior body, the positive electrode 11 a, andthe negative electrode 11 b are less likely to be damaged and thebattery characteristics are less likely to deteriorate even when thebattery 50 is repeatedly bent and unbent. When the positive electrodeactive material described in the above embodiment is used for thepositive electrode 11 a included in the battery 50, a battery with moreexcellent cycle characteristics can be obtained.

Embodiment 4

In this embodiment, examples of electronic devices including thesecondary battery of one embodiment of the present invention aredescribed.

First, FIGS. 24A to 24G show examples of electronic devices includingthe bendable secondary battery described in Embodiment 3. Examples of anelectronic device including a flexible secondary battery includetelevision sets (also referred to as televisions or televisionreceivers), monitors of computers or the like, digital cameras ordigital video cameras, digital photo frames, mobile phones (alsoreferred to as cellular phones or mobile phone devices), portable gamemachines, portable information terminals, audio reproducing devices, andlarge game machines such as pachinko machines.

In addition, a flexible secondary battery can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of an automobile.

FIG. 24A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a secondary battery 7407. When the secondary battery of oneembodiment of the present invention is used as the secondary battery7407, a lightweight mobile phone with a long lifetime can be provided.

FIG. 24B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is curved by external force, the secondary battery7407 included in the mobile phone 7400 is also curved. FIG. 24Cillustrates the curved secondary battery 7407. The secondary battery7407 is a thin storage battery. The secondary battery 7407 is curved andfixed. Note that the secondary battery 7407 includes a lead electrode7408 electrically connected to a current collector 7409. The currentcollector 7409 is, for example, copper foil, and partly alloyed withgallium; thus, adhesion between the current collector 7409 and an activematerial layer in contact with the current collector 7409 is improvedand the secondary battery 7407 can have high reliability even in a stateof being bent.

FIG. 24D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a secondary battery 7104. FIG. 24Eillustrates the bent secondary battery 7104. When the curved secondarybattery 7104 is on a user's arm, the housing changes its form and thecurvature of a part or the whole of the secondary battery 7104 ischanged. Note that the radius of curvature of a curve at a point refersto the radius of the circular arc that best approximates the curve atthat point. The reciprocal of the radius of curvature is curvature.Specifically, part or the whole of the housing or the main surface ofthe secondary battery 7104 is changed in the range of radius ofcurvature from 40 mm to 150 mm. When the radius of curvature at the mainsurface of the secondary battery 7104 is greater than or equal to 40 mmand less than or equal to 150 mm, the reliability can be kept high. Whenthe secondary battery of one embodiment of the present invention is usedas the secondary battery 7104, a lightweight portable display devicewith a long lifetime can be provided.

FIG. 24F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power on/off, on/off of wireless communication, setting andcancellation of a silent mode, and setting and cancellation of a powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by setting the operation systemincorporated in the portable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. For example, mutual communication between theportable information terminal 7200 and a headset capable of wirelesscommunication can be performed, and thus hands-free calling is possible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200includes the secondary battery of one embodiment of the presentinvention. When the secondary battery of one embodiment of the presentinvention is used, a lightweight portable information terminal with along lifetime can be provided. For example, the secondary battery 7104illustrated in FIG. 24E that is in the state of being curved can beprovided in the housing 7201. Alternatively, the secondary battery 7104illustrated in FIG. 24E can be provided in the band 7203 such that itcan be curved.

A portable information terminal 7200 preferably includes a sensor. Asthe sensor, for example a human body sensor such as a fingerprintsensor, a pulse sensor, or a temperature sensor, a touch sensor, apressure sensitive sensor, an acceleration sensor, or the like ispreferably mounted.

FIG. 24G illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the secondary battery ofone embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

When the secondary battery of one embodiment of the present invention isused as the secondary battery included in the display device 7300, alightweight display device with a long lifetime can be provided.

In addition, FIG. 24H, FIGS. 25A to 25C, and FIG. 26 show examples ofelectronic devices including the secondary battery with excellent cyclecharacteristics described in the above embodiment.

When the secondary battery of one embodiment of the present invention isused as a secondary battery of a daily electronic device, a lightweightproduct with a long lifetime can be provided. As the daily electronicdevices, an electric toothbrush, an electric shaver, electric beautyequipment, and the like are given. As secondary batteries of theseproducts, in consideration of handling ease for users, small andlightweight stick type secondary batteries with high capacity aredesired.

FIG. 24H is a perspective view of a device which is called a vaporizer.In FIG. 24H, a vaporizer 7500 includes an atomizer 7501 including aheating element, a secondary battery 7504 supplying power to theatomizer, and a cartridge 7502 including a liquid supply bottle, asensor, and the like. To improve safety, a protection circuit whichprevents overcharge and overdischarge of the secondary battery 7504 maybe electrically connected to the secondary battery 7504. The secondarybattery 7504 in FIG. 24H includes an output terminal for connecting to acharger. When the vaporizer 7500 is held by a user, the secondarybattery 7504 becomes a tip portion; thus, it is preferable that thesecondary battery 7504 have a short total length and be lightweight.With the secondary battery of one embodiment of the present inventionwhich has high capacity and excellent cycle characteristics, the smalland lightweight vaporizer 7500 which can be used for a long time for along period can be provided.

Next, FIGS. 25A and 25B illustrate an example of a foldable tabletterminal. A tablet terminal 9600 illustrated in FIGS. 25A and 25Bincludes a housing 9630 a, a housing 9630 b, a movable portion 9640connecting the housings 9630 a and 9630 b, a display portion 9631including a display portion 9631 a and a display portion 9631 b, adisplay mode changing switch 9626, a power switch 9627, a power savingmode changing switch 9625, a fastener 9629, and an operation switch9628. A flexible panel is used for the display portion 9631, whereby atablet terminal with a larger display portion can be provided. FIG. 25Aillustrates the tablet terminal 9600 that is opened, and FIG. 25Billustrates the tablet terminal 9600 that is closed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630 a and 9630 b. The power storage unit 9635 is providedacross the housings 9630 a and 9630 b, passing through the movableportion 9640.

Part of the display portion 9631 a can be a touch panel region and datacan be input when a displayed operation key is touched. Although astructure in which a half region in the display portion 9631 a has onlya display function and the other half region has a touch panel functionis shown as an example, the display portion 9631 a is not limited to thestructure. The whole region in the display portion 9631 a may have atouch panel function. For example, the display portion 9631 a candisplay keyboard buttons in the whole region to be a touch panel, andthe display portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region. A switching button for showing/hiding akeyboard of the touch panel is touched with a finger, a stylus, or thelike, so that keyboard buttons can be displayed on the display portion9631 b.

Touch input can be performed in the touch panel region and the touchpanel region at the same time.

The display mode switch 9626 can switch the display between a portraitmode and a landscape mode, and between monochrome display and colordisplay, for example. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. Anotherdetection device including a sensor for detecting inclination, such as agyroscope sensor or an acceleration sensor, may be incorporated in thetablet terminal, in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same area in FIG. 25A, one embodiment of the present invention isnot limited to this example. The display portion 9631 a and the displayportion 9631 b may have different areas or different display quality.For example, one display panel may be capable of higher-definitiondisplay than the other display panel.

The tablet terminal is closed in FIG. 25B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DCDC converter 9636. The power storage unit ofone embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded such that the housings 9630 a and9630 b overlap with each other when not in use. Thus, the displayportions 9631 a and 9631 b can be protected, which increases thedurability of the tablet terminal 9600. With the power storage unit 9635including the secondary battery of one embodiment of the presentinvention which has high capacity and excellent cycle characteristics,the tablet terminal 9600 which can be used for a long time for a longperiod can be provided.

The tablet terminal illustrated in FIGS. 25A and 25B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, or the time on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630 and thepower storage unit 9635 can be charged efficiently. The use of alithium-ion battery as the power storage unit 9635 brings an advantagesuch as reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 25B will be described with reference to a blockdiagram in FIG. 25C. The solar cell 9633, the power storage unit 9635,the DC-DC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 25C, and the power storageunit 9635, the DC-DC converter 9636, the converter 9637, and theswitches SW1 to SW3 correspond to the charge and discharge controlcircuit 9634 in FIG. 25B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDCDC converter 9636 to a voltage for charging the power storage unit9635. When the power from the solar cell 9633 is used for the operationof the display portion 9631, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 to a voltageneeded for operating the display portion 9631. When display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on, so that the power storage unit 9635 can becharged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module that transmits and receives power wirelessly(without contact) to charge the battery or with a combination of othercharging means.

FIG. 26 illustrates other examples of electronic devices. In FIG. 26, adisplay device 8000 is an example of an electronic device including asecondary battery 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, the secondary battery 8004, and thelike. The secondary battery 8004 of one embodiment of the presentinvention is provided in the housing 8001. The display device 8000 canreceive electric power from a commercial power supply. Alternatively,the display device 8000 can use electric power stored in the secondarybattery 8004. Thus, the display device 8000 can operate with the use ofthe secondary battery 8004 of one embodiment of the present invention asan uninterruptible power supply even when electric power cannot besupplied from a commercial power supply due to power failure or thelike.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 26, an installation lighting device 8100 is an example of anelectronic device using a secondary battery 8103 of one embodiment ofthe present invention. Specifically, the lighting device 8100 includes ahousing 8101, a light source 8102, the secondary battery 8103, and thelike. Although FIG. 26 illustrates the case where the secondary battery8103 is provided in a ceiling 8104 on which the housing 8101 and thelight source 8102 are installed, the secondary battery 8103 may beprovided in the housing 8101. The lighting device 8100 can receiveelectric power from a commercial power supply. Alternatively, thelighting device 8100 can use electric power stored in the secondarybattery 8103. Thus, the lighting device 8100 can operate with the use ofthe secondary battery 8103 of one embodiment of the present invention asan uninterruptible power supply even when electric power cannot besupplied from a commercial power supply due to power failure or thelike.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 26 as an example, the secondarybattery of one embodiment of the present invention can be used as aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the secondary battery can be used in a tabletop lightingdevice or the like.

As the light source 8102, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and a light-emittingelement such as an LED or an organic EL element are given as examples ofthe artificial light source.

In FIG. 26, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including asecondary battery 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, the secondary battery 8203, and the like. Although FIG. 26illustrates the case where the secondary battery 8203 is provided in theindoor unit 8200, the secondary battery 8203 may be provided in theoutdoor unit 8204. Alternatively, the secondary batteries 8203 may beprovided in both the indoor unit 8200 and the outdoor unit 8204. The airconditioner can receive electric power from a commercial power supply.Alternatively, the air conditioner can use electric power stored in thesecondary battery 8203. Particularly in the case where the secondarybatteries 8203 are provided in both the indoor unit 8200 and the outdoorunit 8204, the air conditioner can operate with the use of the secondarybattery 8203 of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 26 as an example, thesecondary battery of one embodiment of the present invention can be usedin an air conditioner in which the functions of an indoor unit and anoutdoor unit are integrated in one housing.

In FIG. 26, an electric refrigerator-freezer 8300 is an example of anelectronic device using a secondary battery 8304 of one embodiment ofthe present invention. Specifically, the electric refrigerator-freezer8300 includes a housing 8301, a refrigerator door 8302, a freezer door8303, the secondary battery 8304, and the like. The secondary battery8304 is provided in the housing 8301 in FIG. 26. The electricrefrigerator-freezer 8300 can receive electric power from a commercialpower supply. Alternatively, the electric refrigerator-freezer 8300 canuse electric power stored in the secondary battery 8304. Thus, theelectric refrigerator-freezer 8300 can operate with the use of thesecondary battery 8304 of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that among the electronic devices described above, a high-frequencyheating apparatus such as a microwave oven and an electronic device suchas an electric rice cooker require high power in a short time. Thetripping of a circuit breaker of a commercial power source in use ofelectronic devices can be prevented by using the secondary battery ofone embodiment of the present invention as an auxiliary power source forsupplying power which cannot be supplied enough by a commercial powersource.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of power which isactually used to the total amount of power which can be supplied from acommercial power source (such a proportion referred to as a usage rateof power) is low, power can be stored in the secondary battery, wherebythe usage rate of power can be reduced in a time period when theelectronic devices are used. For example, in the case of the electricrefrigerator-freezer 8300, power can be stored in the secondary battery8304 in night time when the temperature is low and the refrigerator door8302 and the freezer door 8303 are not often opened and closed. On theother hand, in daytime when the temperature is high and the refrigeratordoor 8302 and the freezer door 8303 are frequently opened and closed,the secondary battery 8304 is used as an auxiliary power source; thus,the usage rate of power in daytime can be reduced.

According to one embodiment of the present invention, the secondarybattery can have excellent cycle characteristics and improvereliability. Furthermore, in accordance with one embodiment of thepresent invention, a secondary battery with high capacity can beobtained; thus, the secondary battery itself can be made more compactand lightweight as a result of improved characteristics of the secondarybattery. Thus, the secondary battery of one embodiment of the presentinvention is used in the electronic device described in this embodiment,whereby a more lightweight electronic device with a longer lifetime canbe obtained. This embodiment can be implemented in appropriatecombination with any of the other embodiments.

Embodiment 5

In this embodiment, examples of vehicles including the secondary batteryof one embodiment of the present invention are described.

The use of secondary batteries in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 27A to 27C each illustrate an example of a vehicle using thesecondary battery of one embodiment of the present invention. Anautomobile 8400 illustrated in FIG. 27A is an electric vehicle that runson the power of an electric motor. Alternatively, the automobile 8400 isa hybrid electric vehicle capable of driving appropriately using eitheran electric motor or an engine. One embodiment of the present inventioncan provide a high-mileage vehicle. The automobile 8400 includes thesecondary battery. As the secondary battery, the modules of thesecondary batteries illustrated in FIGS. 12C and 12D may be arranged tobe used in a floor portion in the automobile. Alternatively, a batterypack in which a plurality of secondary batteries each of which isillustrated in FIGS. 17A to 17C are combined may be placed in a floorportion in the automobile. The secondary battery is used not only fordriving an electric motor 8406, but also for supplying electric power toa light-emitting device such as a headlight 8401 or a room light (notillustrated).

The secondary battery can also supply electric power to a display deviceof a speedometer, a tachometer, or the like included in the automobile8400. Furthermore, the secondary battery can supply electric power to asemiconductor device included in the automobile 8400, such as anavigation system.

FIG. 27B illustrates an automobile 8500 including the secondary battery.The automobile 8500 can be charged when the secondary battery issupplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.27B, secondary batteries 8024 and 8025 included in the automobile 8500is charged with the use of a ground-based charging apparatus 8021through a cable 8022. In charging, a given method such as CHAdeMO(registered trademark) or Combined Charging System may be employed as acharging method, the standard of a connector, or the like asappropriate. The ground-based charging apparatus 8021 may be a chargingstation provided in a commerce facility or a power source in a house.With the use of a plug-in technique, the secondary battery 8024 includedin the automobile 8500 can be charged by being supplied with electricpower from the outside, for example. The charging can be performed byconverting AC electric power into DC electric power through a convertersuch as an AC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. A solar cell may be provided in the exterior of theautomobile to charge the secondary battery when the automobile stops ormoves. To supply electric power in such a contactless manner, anelectromagnetic induction method or a magnetic resonance method can beused.

FIG. 27C shows an example of a motorcycle using the secondary battery ofone embodiment of the present invention. A motor scooter 8600illustrated in FIG. 27C includes a secondary battery 8602, side mirrors8601, and indicators 8603. The secondary battery 8602 can supplyelectric power to the indicators 8603.

Furthermore, in the motor scooter 8600 illustrated in FIG. 27C, thesecondary battery 8602 can be held in a storage unit under seat 8604.The secondary battery 8602 can be held in the storage unit under seat8604 even with a small size. The secondary battery 8602 is detachable;thus, the secondary battery 8602 is carried indoors when it is charged,and is stored before the motorcycle is driven.

In accordance with one embodiment of the present invention, thesecondary battery can have improved cycle characteristics and thecapacity of the secondary battery can be increased. Thus, the secondarybattery itself can be made more compact and lightweight. The compact andlightweight secondary battery contributes to a reduction in the weightof a vehicle, and thus increases the driving radius. Furthermore, thesecondary battery included in the vehicle can be used as a power sourcefor supplying electric power to products other than the vehicle. In sucha case, the use of a commercial power source can be avoided at peak timeof electric power demand, for example. If the use of a commercial powersource can be avoided at peak time of electric power demand, theavoidance can contribute to energy saving and a reduction in carbondioxide emissions. Moreover, if the cycle characteristics are excellent,the secondary battery can be used for a long period; thus, the useamount of rare metals such as cobalt can be reduced.

This embodiment can be implemented in appropriate combination with theother embodiments.

Example 1

In this example, the positive electrode active materials which areembodiments of the present invention are formed, and the observationresults of the positive electrode active materials by STEM, the resultsof TEM images subjected to fast Fourier transform, and the analysisresults obtained by energy dispersive X-ray spectroscopy (EDX) aredescribed. In addition, the evaluation results of characteristics ofsecondary batteries containing the positive electrode active materialsare described.

[Formation of Positive Electrode Active Material] <<Sample 01>>

In this example, a positive electrode active material of Sample 01,which contains lithium cobaltate as a composite oxide of lithium and afirst transition metal contained in a first region, lithium titanate asan oxide of a second transition metal contained in a second region, andmagnesium oxide as an oxide of a representative element contained in athird region, was formed.

In this example, lithium cobalt oxide particles (C-20F, produced byNIPPON CHEMICAL INDUSTRIAL CO., LTD.) were used as a starting material.Thus, in this example, Step 12 and Step 13 described in Embodiment 1were omitted. Note that the above-described lithium cobalt oxideparticles each have a particle diameter of approximately 20 μm, andcontain fluorine, magnesium, calcium, sodium, silicon, sulfur, andphosphorus in a region which can be analyzed by XPS.

Next, as Step 14, the lithium cobalt oxide particles containingmagnesium and fluorine were coated with a material containing titaniumby a sol-gel method. Specifically, TTIP was dissolved in isopropanol,and an isopropanol solution of TTIP was formed. Then, the lithium cobaltoxide particles were mixed into the solution so that TTIP to lithiumcobalt oxide containing magnesium and fluorine was 0.01 ml/g.

The above mixed solution was stirred with a magnetic stirrer for fourhours, at 25° C., and at a humidity of 90% RH. Through the process,water in an atmosphere and TTIP caused hydrolysis and polycondensationreaction, and a layer containing titanium was formed on the surface ofthe lithium cobalt oxide particle containing magnesium and fluorine.

The mixed solution which had been subjected to the above process wasfiltered to collect the residue. As a filter for filtration, Kiriyamafilter paper (No. 4) was used.

The collected residue was dried in a vacuum at 70° C. for one hour.

Next, the lithium cobalt oxide particles coated with the materialcontaining titanium was heated. With a muffle furnace, the heating wasperformed under the following conditions: the flow rate of dry air was10 L/min; the temperature was 800° C. (the temperature rising rate was200° C./h); and the retention time was two hours. The dew point of thedry air is preferably lower than or equal to −109° C.

Then, the heated particles were cooled to room temperature. The time ofdecreasing temperature from the retention temperature to roomtemperature was 10 hours to 15 hours. After that, crushing treatment wasperformed. In the crushing treatment, the particles were made to passthrough a sieve. The sieve has an aperture width of 53 μm.

Lastly, the cooled particles were collected, and the positive electrodeactive material of Sample 01 was obtained.

<<Sample 02>>

Sample 02 was formed as a comparative example by heating lithium cobaltoxide particles containing magnesium and fluorine without being coatedwith a material containing titanium.

Lithium cobalt oxide particles produced by NIPPON CHEMICAL INDUSTRIALCO., LTD. (product name: C-20F) were used as the lithium cobalt oxideparticles containing magnesium and fluorine.

The lithium cobalt oxide particles containing magnesium and fluorinewere heated. The heating was performed under the following conditions:the temperature was 800° C. (the temperature rising rate was 200° C./h);the retention time was two hours; and the flow rate of oxygen was 10L/min.

The heated particles were cooled and made to pass through a sieve likeSample 01 to obtain a positive electrode active material of Sample 02.

It is probable that Sample 02 is a positive electrode active materialwhich contains lithium cobalt oxide inside and includes a regioncontaining magnesium in a superficial portion.

<<Sample 03>>

Sample 03 was formed as a comparative example in the following manner: aregion containing titanium was formed in lithium cobalt oxide particleswhich do not contain magnesium by a sol-gel method and then the lithiumcobalt oxide particles were heated.

Lithium cobalt oxide particles produced by NIPPON CHEMICAL INDUSTRIALCO., LTD. (product name: C-10N) were used. In the lithium cobalt oxideparticles, magnesium is not detected and fluorine is detected atapproximately 1 atomic % by XPS.

A region containing titanium was formed by a sol-gel method in thelithium cobalt oxide particles, and the lithium cobalt oxide particleswere dried, heated, cooled, and made to pass through a sieve, likeSample 01. The obtained lithium cobalt oxide particles were used as apositive electrode active material of Sample 03.

It is probable that Sample 03 is a positive electrode active materialwhich contains lithium cobalt oxide inside and includes a regioncontaining titanium in a superficial portion.

<<Sample 04>>

For Sample 04, as a comparative example, lithium cobalt oxide particleswere used as it is without being heated.

Lithium cobalt oxide particles produced by NIPPON CHEMICAL INDUSTRIALCO., LTD. (product name: C-10N) were used.

Sample 04 is a positive electrode active material which does not have acoating layer.

<<Sample 05>>

For Sample 05, as a comparative example, lithium cobalt oxide particlescontaining magnesium and fluorine were used as it is without beingheated.

Lithium cobalt oxide particles produced by NIPPON CHEMICAL INDUSTRIALCO., LTD. (product name: C-20F) were used as the lithium cobalt oxideparticles containing magnesium and fluorine. That is, Sample 05 was usedas the same as the starting material of Sample 01.

Table 1 shows the conditions of Sample 01 to Sample 05.

TABLE 1 Conditions Sample 01 LiCoO₂ + Mg + F, coated with a materialcontaining Ti, heated Sample 02 LiCoO₂ + Mg + F, heated Sample 03LiCoO₂, coated with a material containing Ti, heated Sample 04 LiCoO₂,not heated Sample 05 LiCoO₂ + Mg + F, not heated

[STEM]

The obtained positive electrode active material of Sample 01 wasobserved by an electron microscope (JEM-ARM200F, manufactured by JEOLLtd.) under the condition where the acceleration voltage was 200 kV.FIG. 28 shows the obtained electron microscope image. As shown in FIG.28, the positive electrode active material probably includes threedifferent regions: the first region 101; the second region 102; and thethird region 103. The third region 103 is observed as a region brighterthan the first region 101 and the second region 102. Furthermore,crystal orientations of the first region 101 and the second region 102are partly aligned with each other, and crystal orientations of thesecond region 102 and the third region 103 are partly aligned with eachother.

[STEM-FFT]

FIG. 29A1 shows a fast Fourier transform (FFT) image of a region 103FFTin the STEM image of FIG. 28. In FIG. 29A2, a center point O of FIG.29A1 is shown by a cross, and bright points A, B, and C are eachsurrounded by a circle. Similarly, FIG. 29B1 shows an FFT image of aregion 102FFT. In FIG. 29B2, a center point O of FIG. 29B1 is shown by across, and bright points A, B, and C are each surrounded by a circle. Inaddition, FIG. 29C1 shows an FFT image of a region 101FFT. In FIG. 29C2,a center point O of FIG. 29C1 is shown by a cross, and bright points A,B, and C are each surrounded by a circle.

In FIG. 29A2, a distance d between the bright point A and the centerpoint O is 0.256 nm, a distance d between the bright point B and thecenter point O is 0.241 nm, and a distance d between the bright point Cand the center point O is 0.209 nm. In addition, ∠COA is 121°, ∠COB is52°, and ∠AOB is 69°. From these results, the region 103FFT probablycontains magnesium oxide (MgO, cubic crystal).

Similarly, in FIG. 29B2, the distance d between the bright point A andthe center point O is 0.238 nm, the distance d between the bright pointB and the center point O is 0.225 nm, and the distance d between thebright point C and the center point O is 0.198 nm. In addition, ∠COA is123°, ∠COB is 52°, and ∠AOB is 71°. From these results, the region102FFT probably contains lithium titanate (LiTiO₂, cubic crystal).

In FIG. 29C2, the distance d between the bright point A and the centerpoint O is 0.240 nm, the distance d between the bright point B and thecenter point O is 0.235 nm, and the distance d between the bright pointC and the center point O is 0.196 nm. In addition, ∠COA is 126°, ∠COB is52°, and ∠AOB is 74°. From these results, the region 101FFT probablycontains lithium cobaltate (LiCoO₂, rhombohedral).

[EDX]

FIGS. 30A1, 30A2, 30B1, 30B2, 30C1, and 30C2 show a high-angle annulardark field scanning transmission electron microscopy (HAADF-STEM) imageand element mapping images with EDX of the positive electrode activematerial of Sample 01. FIG. 30A1 shows a HAADF-STEM image, FIG. 30A2shows a mapping image of oxygen atoms, FIG. 30B1 shows a mapping imageof cobalt atoms, FIG. 30B2 shows a mapping image of fluorine atoms, FIG.30C1 shows a mapping image of titanium atoms, and FIG. 30C2 shows amapping image of magnesium atoms. Note that in EDX element mappingimages in FIGS. 30A2, 30B1, 30B2, 30C1, and 30C2 and FIGS. 31A2, 31B1,31B2, 31C1, and 31C2, a region where the number of elements is less thanor equal to a lower limit of the detection is indicated in white, and asthe number of elements is increased, the white region becomes black.

As shown in FIGS. 30A2 and 30B1, it is found that the oxygen atoms andthe cobalt atoms are distributed in the whole of the positive electrodeactive material particle. In contrast, as shown in FIGS. 30B2, 30C1, and30C2, it is found that the fluorine atoms, the titanium atoms, and themagnesium atoms are unevenly distributed in a region close to thesurface of the positive electrode active material.

Next, FIGS. 31A1, 31A2, 31B1, 31B2, 31C1, and 31C2 show a HAADF-STEMimage and element mapping images with EDX of the positive electrodeactive material of Sample 05, which is a comparative example. FIG. 31A1shows a HAADF-STEM image, FIG. 31A2 shows a mapping image of oxygenatoms, FIG. 31B1 shows a mapping image of cobalt atoms, FIG. 31B2 showsa mapping image of fluorine atoms, FIG. 31C1 shows a mapping image oftitanium atoms, and FIG. 31C2 shows a mapping image of magnesium atoms.

As shown in FIGS. 31B2 and 31C2, it is found that, even in Sample 05which is not heated, a certain amount of magnesium and fluorine isunevenly distributed in the vicinity of the surface.

[EDX Line Analysis]

FIG. 32 shows results of line analysis with TEM-EDX performed on a crosssection of the vicinity of the surface of the positive electrode activematerial of Sample 01. FIG. 32 is a graph showing data detected on aline connecting the outside of the positive electrode active material ofSample 01 to the inside of the positive electrode active material, and adistance of 0 nm indicates the outside of the positive electrode activematerial and a distance of 14 nm indicates the inside of the particle.With EDX, the analysis region tends to be large, so that elements notonly at a center of an electron beam irradiation region but also in aregion around the center may be detected.

As shown in FIG. 32, it is found that there are peaks of magnesium andtitanium in the vicinity of the surface of the positive electrode activematerial of Sample 01, the distribution of magnesium is closer to thesurface than the distribution of titanium is. It is also found that thepeak of magnesium is closer to the surface than the peak of titanium is.In addition, it is probable that cobalt and oxygen are present from theoutermost surface of the positive electrode active material particle.

As shown in FIG. 32, fluorine is hardly detected. This is probablybecause fluorine, which is a light element, is difficult to detect withEDX.

From the above STEM images, FFT images, element mapping images with EDX,and EDX line analysis, it is found that Sample 01 is a positiveelectrode active material of one embodiment of the present invention,which includes the first region containing lithium cobaltate, the secondregion containing lithium, titanium, cobalt, and oxygen, and the thirdregion containing magnesium and oxygen. It is found that, in Sample 01,part of the second region and part of the third region overlap.

In the graph of FIG. 32, the amount of detected oxygen is stable at adistance of 4 nm or more. Thus, in this example, the average valueO_(ave) of the amount of detected oxygen in the stable region isobtained, and a distance x of the measurement point at which themeasurement value closest to 0.5 O_(ave), the value of 50% of theaverage value O_(ave), is obtained is assumed to be the surface of thepositive electrode active material particle.

In this example, the average value O_(ave) of the amount of detectedoxygen in a range from a distance of 4 nm to a distance of 14 nm is674.2. The x axis of the measurement point at which the measurementvalue closest to 337.1, which is 50% of 674.2, is obtained indicates adistance of 1.71 nm. Thus, in this example, a distance of 1.71 nm in thegraph of FIG. 32 is assumed to be the surface of the positive electrodeactive material particle.

When the surface of the positive electrode active material particle isset at a distance of 1.71 nm in FIG. 32, the peak of magnesium and thepeak of titanium are present at 0.72 nm and 1.00 nm, respectively, fromthe surface of the positive electrode active material particle.

The concentration of magnesium is higher than or equal to ⅕ of the peakfrom the surface of the positive electrode active material particle to adistance of 4.42 nm, that is, to a region at 2.71 nm from the surface.The measurement value of magnesium is less than ⅕ of the peak at adistance of 4.57 nm or more, that is, at a depth of 2.86 nm or more fromthe surface of the positive electrode active material particle. Thus, itis found that in Sample 01, a region from the surface to a depth of 2.71nm is the third region.

Furthermore, the concentration of titanium is higher than or equal to ½of the peak from a distance of 2.14 nm to a distance of 3.42 nm. Thus,it is found that a region from a depth of 0.43 nm to a depth of 1.71 nmfrom the surface of the positive electrode active material particle isthe second region.

Next, evaluation results of charge and discharge characteristics ofsecondary batteries which are fabricated using the positive electrodeactive materials of Sample 01 to Sample 05 formed in the above mannerare described.

[Fabrication of Secondary Batteries]

CR2032 coin-type secondary batteries (with a diameter of 20 mm and aheight of 3.2 mm) were fabricated using the positive electrode activematerials of Sample 01 to Sample 05 formed in the above manner.

A positive electrode formed by applying slurry in which a positiveelectrode active material (LCO), acetylene black (AB), andpolyvinylidene fluoride (PVDF) were mixed at a weight ratio of95:2.5:2.5 to a current collector was used.

A lithium metal was used for the counter electrode.

As an electrolyte contained in an electrolyte solution, 1 mol/L lithiumhexafluorophosphate (LiPF₆) was used, and as the electrolyte solution, asolution in which ethylene carbonate (EC) and diethyl carbonate (DEC) ata volume ratio of 3:7 and vinylene carbonate (VC) at a 2 weight % weremixed was used.

A positive electrode can and a negative electrode can were formed ofstainless steel (SUS).

[Evaluation of Charge and Discharge Characteristics]

Next, charge and discharge characteristics of the secondary batteries ofSample 01 and Sample 05 formed in the above manner were evaluated. Themeasurement temperature was 25° C. Twenty cycles of charging anddischarging were performed at 4.6 V (CCCV, 0.5 C, a cutoff current of0.01 C) and 2.5 V (CC, 0.5 C), respectively. Here, 1C was set to 137mA/g, which was a current value per weight of the positive electrodeactive material.

FIG. 33 is a graph showing charge and discharge characteristics of thesecondary battery using the positive electrode active material of Sample01. FIG. 33 shows excellent charge and discharge characteristics with awide plateau. In addition, results of 20 cycles of charging anddischarging almost overlap, which means that the cycle characteristicsare excellent.

FIG. 34 is a graph showing charge and discharge characteristics of thesecondary battery of Sample 05, which is a comparative example. In theinitial cycles, excellent charge and discharge characteristics areshown; however, as indicated by arrows in FIG. 34, charge capacity anddischarge capacity decrease with an increase in cycles.

[Evaluation of Cycle Characteristics] <<Charging at 4.4 V>>

The cycle characteristics of the secondary batteries of Sample 01 andSample 05 charged at 4.4 V were evaluated. The measurement temperaturewas 25° C. The charging was performed at 4.4 V (CCCV, 0.5 C, a cutoffcurrent of 0.01 C), and the discharging was performed at 2.5 V (CC, 0.5C).

FIG. 35 is a graph showing the cycle characteristics of the secondarybatteries charged at 4.4 V. In FIG. 35, a solid line and a dotted lineindicate secondary batteries containing the positive electrode activematerials of Sample 01 and Sample 05, respectively. As shown in FIG. 35,in the secondary battery containing Sample 01, an energy densityretention rate is 99.5% even after 50 cycles of charging and dischargingwere performed, which shows extremely excellent cycle characteristics.In the secondary battery containing Sample 05, an energy densityretention rate is 94.3% after 50 cycles were performed.

<<Charging at 4.6 V>>

The cycle characteristics of the secondary batteries of Sample 01 toSample 04 charged at 4.6 V were evaluated. The measurement temperaturewas 25° C. The charging was performed to 4.6 V (CCCV, 0.5 C, a cutoffcurrent of 0.01 C), and the discharging was performed to 2.5 V (CC, 0.5C).

FIG. 36 is a graph showing the cycle characteristics charged at 4.6 V.As shown in FIG. 36, in the secondary battery containing Sample 01,which is the positive electrode active material of one embodiment of thepresent invention, an energy density retention rate is 94.1% even after50 cycles of charging and discharging were performed at a high voltageof 4.6 V, which shows extremely excellent cycle characteristics. On theother hand, the secondary batteries containing the positive electrodeactive materials of Sample 02, Sample 03, and Sample 04, which arecomparative examples, are inferior to that of Sample 01, and in Sample04, for example, energy density retention rate is 33.2% after 50 cycleswere performed.

As described above, it is found that the positive electrode activematerial with the structure of one embodiment of the present inventioncan achieve an advantageous effect when charging and discharging isperformed at a voltage higher than 4.4 V.

Example 2

In this example, the positive electrode active materials which areembodiments of the present invention are formed, and results of analysiswhich is different from that in Example 1 are described. In addition,evaluation results of characteristics of secondary batteries containingthe positive electrode active materials under conditions different fromthose in Example 1 are described.

In this example, a positive electrode active material which containslithium cobaltate as a composite oxide of lithium and a first transitionmetal contained in a first region, lithium titanate as an oxide of asecond transition metal contained in a second region, and magnesiumoxide as an oxide of a representative element contained in a thirdregion, was formed.

[Formation of Positive Electrode Active Material and Fabrication ofSecondary Battery] <<Sample 06 and Sample 07>>

In this example, lithium cobalt oxide particles (C-20F, produced byNIPPON CHEMICAL INDUSTRIAL CO., LTD.) were used as a starting material.

Next, as Step 14, the lithium cobalt oxide particles were coated withtitanium oxide by a sol-gel method and dried. Step 14 was performed in amanner similar to that in Example 1 except that mixture was performed sothat TTIP to lithium cobalt oxide was 0.004 ml/g. The lithium cobaltoxide particles which are coated with the titanium oxide and are notheated yet are referred to as Sample 06.

Next, Sample 06, which is the lithium cobalt oxide particles coated withthe titanium oxide, was heated. With a muffle furnace, the heating wasperformed at 800° C. in an oxygen atmosphere under the followingconditions: the retention time was two hours; and the flow rate ofoxygen was 10 L/min.

Then, as in Example 1, the particles were cooled and collected to obtainthe positive electrode active material. The heated positive electrodeactive material is referred to as Sample 07.

[TEM-EDX]

Sample 06 and Sample 07, in particular, cracks generated in theparticles and the vicinity of the cracks were subjected to analysis withTEM-EDX.

First, results of TEM-EDX plane analysis of titanium are shown in FIGS.37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2, 37E1, and 37E2 and FIGS. 38A,38B1, 38B2, 38C1, 38C2, 38D1, 38D2, 38E1, and 38E2.

FIGS. 37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2, 37E1, and 37E2 showTEM-EDX analysis results of Sample 06 before heating. FIG. 37A is across-sectional TEM image showing the surfaces of the particles and thecrack portions. FIG. 37B1 and FIG. 37B2 show a HAADF-STEM image and a Timapping image of a region including the surface of the particle that isdenoted by a circle marked with “1” in FIG. 37A, respectively.Similarly, FIG. 37C1 and FIG. 37C2 show a HAADF-STEM image and a Timapping image of a region at a depth of approximately 20 nm from thesurface in the crack portion that is denoted by a circle marked with “2”in FIG. 37A, respectively. FIG. 37D1 and FIG. 37D2 show a HAADF-STEMimage and a Ti mapping image of a region at a depth of approximately 500nm from the surface in the crack portion that is denoted by a circlemarked with “3” in FIG. 37A, respectively. FIG. 37E1 and FIG. 37E2 showa HAADF-STEM image and a Ti mapping image of a region at a depth ofapproximately 1000 nm from the surface in the crack portion that isdenoted by a circle marked with “4” in FIG. 37A, respectively. Note thatin EDX element mapping images in FIGS. 37A, 37B1, 37B2, 37C1, 37C2,37D1, 37D2, 37E1, and 37E2, FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1,38D2, 38E1, and 38E2, FIGS. 39A, 39B1, 39B2, 39C1, 39C2, 39D1, 39D2,39E1, and 39E2, and FIGS. 40A, 40B1, 40B2, 40C1, 40C2, 40D1, 40D2, 40E1,and 40E2, a region where the number of elements is less than or equal toa lower limit of the detection is indicated in white, and as the numberof elements is increased, the white region becomes black.

FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1, 38D2, 38E1, and 38E2 showTEM-EDX analysis results of Sample 07 after heating. FIG. 38A is across-sectional TEM image showing the surfaces of the particles and thecrack portions. FIG. 38B1 and FIG. 38B2 show a HAADF-STEM image and a Timapping image of a region including the surface of the particle that isdenoted by a circle marked with “1” in FIG. 38A, respectively.Similarly, FIG. 38C1 and FIG. 38C2 show a HAADF-STEM image and a Timapping image of a region at a depth of approximately 20 nm from thesurface in the crack portion that is denoted by a circle marked with “2”in FIG. 38A, respectively. FIG. 38D1 and FIG. 38D2 show a HAADF-STEMimage and a Ti mapping image of a region at a depth of approximately 500nm from the surface in the crack portion that is denoted by a circlemarked with “3” in FIG. 38A, respectively. FIG. 38E1 and FIG. 38E2 showa HAADF-STEM image and a Ti mapping image of a region at a depth ofapproximately 1000 nm from the surface in the crack portion that isdenoted by a circle marked with “4” in FIG. 38A, respectively.

As shown in FIGS. 37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2, 37E1, and37E2 and FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1, 38D2, 38E1, and 38E2,in Sample 06 before heating, segregation of titanium on the surfaces ofthe particles is observed; however, no segregation is observed in thecrack portion. In contrast, in Sample 07 after heating, segregation oftitanium is observed both on the surfaces of the particles and in thecrack portion. That is, it is found that titanium is segregated at thesurface of the crack portion by heating.

Next, results of TEM-EDX plane analysis of magnesium are shown in FIGS.39A, 39B1, 39B2, 39C1, 39C2, 39D1, 39D2, 39E1, and 39E2, and FIGS. 40A,40B1, 40B2, 40C1, 40C2, 40D1, 40D2, 40E1, and 40E2.

FIG. 39A is a cross-sectional TEM image of Sample 06, which is the sameas FIG. 37A. FIGS. 39B1, 39C1, 39D1, and 39E1 are HAADF-STEM images,which are the same as FIGS. 37B1, 37C1, 37D1, and 37E1. FIG. 39B2 showsa Mg mapping image of a region which is the same as FIG. 39B1. FIG. 39C2shows a Mg mapping image of a region which is the same as FIG. 39C1.FIG. 39D2 shows a Mg mapping image of a region which is the same as FIG.39D1. FIG. 39E2 shows a Mg mapping image of a region which is the sameas FIG. 39E1.

FIG. 40A is a cross-sectional TEM image of Sample 07, which is the sameas FIG. 38A. FIGS. 40B1, 40C1, 40D1, and 40E1 are HAADF-STEM images,which are the same as FIGS. 38B1, 38C1, 38D1, and 38E1. FIG. 40B2 showsa Mg mapping image of a region which is the same as FIG. 40B1. FIG. 40C2shows a Mg mapping image of a region which is the same as FIG. 40C1.FIG. 40D2 shows a Mg mapping image of a region which is the same as FIG.40D1. FIG. 40E2 shows a Mg mapping image of a region which is the sameas FIG. 40E1.

As shown in FIGS. 39A, 39B1, 39B2, 39C1, 39C2, 39D1, 39D2, 39E1, and39E2, and FIGS. 40A, 40B1, 40B2, 40C1, 40C2, 40D1, 40D2, 40E1, and 40E2,in Sample 06 before heating, segregation of magnesium is not observed onthe surfaces of the particles or in the crack portion. In contrast, inSample 07 after heating, segregation of magnesium is observed both onthe surfaces of the particles and in the crack portion.

Next, to quantify titanium and magnesium, EDX point analysis wasperformed on the regions indicated by circles marked with 1 to 6 in FIG.37A and the regions indicated by circles marked with 1 to 6 in FIG. 38A.In each region, two points were measured.

FIGS. 41A and 41B show results of EDX point analysis in an atomic ratioof titanium to cobalt. FIG. 41A shows results of Sample 06 beforeheating. Detection points 1 to 6 in FIG. 41A correspond to the regionsindicated by circles marked with 1 to 6 in FIG. 37A. FIG. 41B showsresults of Sample 07 after heating. Detection points 1 to 6 in FIG. 41Bcorrespond to the regions indicated by circles marked with 1 to 6 inFIG. 38A.

As shown in FIGS. 41A and 41B, in the crack portion of Sample 06, Ti/Cois less than or equal to 0.01 in each measurement point; in contrast, inthe crack portion of Sample 07, the amount of titanium is increased inmany points, and there are measurement points where Ti/Co is greaterthan or equal to 0.05. Furthermore, Ti/Co on the surfaces of theparticles of Sample 07 is between 0.10 and 0.18.

Next, FIGS. 42A and 42B show results of EDX point analysis in an atomicratio of magnesium to cobalt. The detection points are the same as thosein FIGS. 41A and 41B.

As shown in FIGS. 42A and 42B, in Sample 06, Mg/Co is less than or equalto 0.03 both on the surfaces of the particles and in the crack portion;in contrast, in Sample 07, there are many points where the amount ofmagnesium is increased both on the surfaces of the particles and in thecrack portion. Furthermore, Mg/Co on the surfaces of the particles isbetween 0.15 and 0.50, and that in the crack portion is between 0 and0.22.

Next, CR2032 coin-type secondary batteries were fabricated using thepositive electrode active material of Sample 07 after heating. Apositive electrode formed by applying slurry in which a positiveelectrode active material (LCO) of Sample 02, AB, and polyvinylidenefluoride (PVDF) were mixed at a weight ratio of 95:3:2 to a positiveelectrode current collector was used. As the positive electrode currentcollector, 20-μm-thick aluminum foil was used. The amount of positiveelectrode active material layer containing the positive electrode activematerial, AB, and PVDF was 7.6 mg/cm².

A lithium metal was used for the counter electrode.

An electrolyte solution formed in such a manner that 1 mol/L LiPF₆ wasdissolved in a solution in which ethylene carbonate (EC) and diethylcarbonate (DEC) were mixed at a volume ratio of 3:7, and vinylenecarbonate (VC) was added to the solution at a 2 wt % was used.

[Initial Characteristics, Rate Characteristics]

Initial characteristics and rate characteristics of the secondarybattery using the positive electrode active material of Sample 07 formedin the above manner were measured.

In the measurement of the initial characteristics, charging wasperformed at CCCV, 0.2 C, 4.6 V, and a cutoff current of 0.05 C, anddischarging was performed at CC, 0.2 C, and a cutoff voltage of 3.0 V.Here, 1C was set to 160 mA/g, which was a current value per weight ofthe positive electrode active material. The measurement temperature was25° C. Table 2 shows measurement results of initial characteristics.

TABLE 2 Initial charge Initial discharge Initial charge and capacitycapacity discharge efficiency [mAh/g] [mAh/g] [%] 221.2 217.5 98.3

The rate characteristics were measured after the initial characteristicswere measured. The measurement was performed by changing a dischargerate in the following order: 0.2 C charge/0.2 C discharge; 0.2 Ccharge/0.5 C discharge; 0.2 C charge/1.0 C discharge; 0.2 C charge/2.0 Cdischarge; 0.2 C charge/3.0 C discharge; 0.2 C charge/4.0 C discharge;and 0.2 C charge/5.0 C discharge. Note that the conditions other thanthe discharge rate are the same as those of the measurement of theinitial characteristics. The measurement temperature was 25° C.

Table 3 shows the measurement results of the initial characteristics andthe rate characteristics. In addition, FIG. 43 shows discharge curves ofthe rates.

TABLE 3 Discharge capacity Rate Discharge capacity retention rate [C][mAh/g] [%] 0.2 218.2 100.0 0.5 215.8 98.9 1.0 213.1 97.7 2.0 207.9 95.33.0 204.0 93.5 4.0 198.8 91.1 5.0 184.3 84.5

[Temperature Characteristics]

Next, a cell was fabricated under conditions similar to those of a cellfor evaluating the rate characteristics except that the amount ofpositive electrode active material layer was 8.2 mg/cm², and thetemperature characteristics were measured. Charging was performed at 25°C., CCCV, 0.2 C, 4.6 V, and a cutoff current of 0.05 C. Discharging wasperformed at 25° C., 0° C., −10° C., −20° C., and 45° C. in this order,CC, 0.2 C, and a cutoff voltage of 3.0 V. FIG. 44 shows measurementresults of temperature characteristics.

[Cycle Characteristics]

Next, a cell was fabricated under conditions similar to those of thecell for measuring the temperature characteristics, and the cyclecharacteristics were measured. In the measurement of the cyclecharacteristics, charging was performed at CCCV, 1.0 C, 4.55 V, and acutoff current of 0.05 C, and discharging was performed at CC, 1.0 C,and a cutoff voltage of 3.0 V. The measurement temperature of the cyclecharacteristics was 45° C. and 100 cycles were measured. The dischargecapacity retention rate after 100 cycles was 86%. FIG. 45 is a graphshowing the discharge capacity retention rate of the measured cyclecharacteristics.

From the measurement results, the specific surface area of the positiveelectrode active material of Sample 07 was 0.13 m²/g.

Furthermore, from the measurement results of particle size distributionof the positive electrode active material of Sample 07, the averageparticle diameter was 21.5 μm, 10% D was 13.1 μm, 50% D was 22.0 μm, and90% D was 34.4 μm.

The tap density of the positive electrode active material of Sample 07is 2.21 g/cm³. The tap density was measured with MULTI TESTER MT-1000(manufactured by SEISHIN ENTERPRISE Co., Ltd.).

As described above, it is found that the positive electrode activematerial of Sample 07 which is one embodiment of the present inventionshows excellent initial charge and discharge capacity, ratecharacteristics, and cycle characteristics. In particular, the initialcharge and discharge capacity is high, which is 98% or higher; thus, itis probable that a side reaction is inhibited. In addition, even at ahigh discharge rate of 2 C, an excellent capacity of 96.1% is shownusing 0.2 C as a reference.

Example 3

In this example, a positive electrode active material including a regioncontaining titanium and magnesium in a superficial portion was formed bychanging the ratio of Li to the first transition metal of startingmaterials, and evaluation results of characteristics are shown.

[Formation of Positive Electrode Active Material]

In this example, positive electrode active materials of Samples 11 to17, Samples 21 to 28, and Samples 31 to 40 in which cobalt was used asthe first transition metal were prepared. Formation methods andconditions of these samples are as follows.

<<Samples 11 to 17>>

First, a source of lithium, a source of cobalt, a source of magnesium,and a source of fluorine, which are to be starting materials, wereindividually weighed. In this example, lithium carbonate, cobalt oxide,magnesium oxide, and lithium fluoride were used as the source oflithium, the source of cobalt, the source of magnesium, and the sourceof fluorine, respectively.

At that time, the starting materials of Sample 11 were weighed so thatthe ratio of Li to Co was 1.00. The starting materials of Sample 12 wereweighed so that the ratio of Li to Co was 1.03. The starting materialsof Sample 13 were weighed so that the ratio of Li to Co was 1.05. Thestarting materials of Sample 14 were weighed so that the ratio of Li toCo was 1.06. The starting materials of Sample 15 were weighed so thatthe ratio of Li to Co was 1.07. The starting materials of Sample 16 wereweighed so that the ratio of Li to Co was 1.08. The starting materialsof Sample 17 were weighed so that the ratio of Li to Co was 1.13.

In addition, the starting materials of each of Samples 11 to 17 wereweighed so that, when the number of atoms of cobalt contained in thestarting materials was set to 1, the number of atoms of magnesium was0.01 and the number of atoms of fluorine was 0.02.

Next, the weighed starting materials were separately mixed with a ballmill for each sample.

Then, the mixed starting materials were baked. The baking was performedat 1000° C. for 10 hours under the following conditions: the temperaturerising rate was 200° C./h; and the flow rate of dry air was 10 L/min.

Through the above process, particles of a composite oxide containinglithium, cobalt, fluorine, and magnesium were synthesized.

Next, TTIP was added to 2-propanol so that the amount of TTIP per weightof the positive electrode active material was 0.01 ml/g and then mixingwas performed, so that a 2-propanol solution of tetra-i-propoxy titaniumwas formed.

To the 2-propanol solution of TTIP, the particles of a composite oxidecontaining lithium, cobalt, fluorine, and magnesium were added and thenmixing was performed.

The above-described mixed solution was stirred with a magnetic stirrerfor four hours, at 25° C., and at a humidity of 90% RH. Through theprocess, water in an atmosphere and TTIP caused hydrolysis andpolycondensation reaction, and a layer containing titanium was formed onthe surface of the lithium cobalt oxide particle containing magnesiumand fluorine.

The mixed solution which had been subjected to the above process wasfiltered to collect the residue. As a filter for filtration, Kiriyamafilter paper (No. 4) was used.

The collected residue was dried in a vacuum at 70° C. for one hour.

The dried particles were heated. The heating was performed in an oxygenatmosphere under the following conditions: the temperature was 800° C.(the temperature rising rate was 200° C./h); and the retention time wastwo hours.

The heated particles were cooled and subjected to crushing treatment. Inthe crushing treatment, the particles were made to pass through a sieve.The sieve has an aperture width of 53 μm.

The particles which were subjected to crushing treatment were used asthe positive electrode active materials of Samples 11 to 17.

<<Samples 21 to 27>>

The starting materials of Samples 21 to 27 were the same as those ofSamples 11 to 16. At that time, the starting materials of Sample 21 wereweighed so that the ratio of Li to Co was 1.00. The starting materialsof Sample 22 were weighed so that the ratio of Li to Co was 1.03. Thestarting materials of Sample 23 were weighed so that the ratio of Li toCo was 1.05. The starting materials of Sample 24 were weighed so thatthe ratio of Li to Co was 1.06. The starting materials of Sample 25 wereweighed so that the ratio of Li to Co was 1.07. The starting materialsof Sample 26 were weighed so that the ratio of Li to Co was 1.08. Thestarting materials of Sample 27 were weighed so that the ratio of Li toCo was 1.13.

Samples 21 to 27 were formed in manners similar to those of Samples 11to 17 except that the concentration of TTIP in the 2-propanol solutionwas adjusted so that the amount of TTIP per weight of the positiveelectrode active material was 0.02 ml/g.

<<Sample 28>>

The ratio of Li to Co of the starting materials and the amount of TTIPof Sample 28 were the same as the ratio of Li to Co of the startingmaterials and the amount of TTIP of Sample 23. That is, in Sample 28,the starting materials were weighed so that the ratio of Li to Co was1.05, and the amount of TTIP per weight of the positive electrode activematerial was 0.02 ml/g.

Note that in Sample 28, after the starting materials were mixed, bakingwas performed at 950° C.

Sample 28 was formed in a manner similar to that of Sample 23 except forthe baking temperature.

It is probable that Samples 11 to 17 and Samples 21 to 28 are each apositive electrode active material which contains lithium cobaltateinside and includes a region containing titanium and magnesium in asuperficial portion.

<<Samples 31 to 40>>

Samples 31 to 40 were formed as comparative examples, each of which didnot include a region containing titanium.

The starting materials of Sample 31 were weighed so that the ratio of Lito Co was 1.00. The starting materials of Sample 32 were weighed so thatthe ratio of Li to Co was 1.01. The starting materials of Sample 33 wereweighed so that the ratio of Li to Co was 1.02. The starting materialsof Sample 34 were weighed so that the ratio of Li to Co was 1.03. Thestarting materials of Sample 35 were weighed so that the ratio of Li toCo was 1.035. The starting materials of Sample 36 were weighed so thatthe ratio of Li to Co was 1.04. The starting materials of Sample 37 wereweighed so that the ratio of Li to Co was 1.051. The starting materialsof Sample 38 were weighed so that the ratio of Li to Co was 1.061. Thestarting materials of Sample 39 were weighed so that the ratio of Li toCo was 1.081. The starting materials of Sample 40 were weighed so thatthe ratio of Li to Co was 1.130.

In addition, the starting materials of each of Samples 31 to 40 wereweighed so that, when the number of atoms of cobalt contained in thestarting materials was set to 1, the number of atoms of magnesium was0.01 and the number of atoms of fluorine was 0.02.

Next, the weighed starting materials were separately mixed with a ballmill for each sample.

Then, the mixed starting materials were baked. The baking was performedat 1000° C. for 10 hours under the following conditions: the temperaturerising rate was 200° C./h; and the flow rate of dry air was 10 L/min.

Through the above process, particles of a composite oxide containinglithium, cobalt, fluorine, and magnesium were synthesized.

The synthesized particles were cooled and then heated. The heating wasperformed in an oxygen atmosphere under the following conditions: thetemperature was 800° C. (the temperature rising rate was 200° C./h); andthe retention time was two hours.

The heated particles were cooled and subjected to crushing treatment. Inthe crushing treatment, the particles were made to pass through a sieve.The sieve has an aperture width of 53 μm.

The particles which were subjected to crushing treatment were used asthe positive electrode active materials of Samples 31 to 40.

Table 4 shows the formation conditions of Samples 11 to 17, Samples 21to 28, and Samples 31 to 40.

TABLE 4 Baking Li/Co TTIP temperature Sample 11 1.00 0.01 ml/g 1000° C.Sample 12 1.03 Sample 13 1.05 Sample 14 1.06 Sample 15 1.07 Sample 161.08 Sample 17 1.13 Sample 21 1.00 0.02 ml/g 1000° C. Sample 22 1.03Sample 23 1.05 Sample 24 1.06 Sample 25 1.07 Sample 26 1.08 Sample 271.13 Sample 28 1.05 0.02 ml/g  950° C. Sample 31 1.00 — 1000° C. Sample32 1.01 Sample 33 1.02 Sample 34 1.03 Sample 35 1.035 Sample 36 1.04Sample 37 1.051 Sample 38 1.061 Sample 39 1.081 Sample 40 1.130

[XPS]

The positive electrode active materials of Samples 11 to 17, Samples 21to 28, and Samples 31 to 40 were subjected to an XPS analysis. Table 5shows results of the XPS analysis of Samples 11 to 17, Table 6 showsresults of the XPS analysis of Samples 21 to 28, and Table 7 showsresults of the XPS analysis of Samples 31 to 40. Tables 5 to 7 show arelative value of the concentration of each element under the conditionwhere the concentration of cobalt is 1.

TABLE 5 Relative value under condition where concentration of Co is 1Li/Co Li Co O C F Mg Ti Si Ca Na S Sample 11 1.00 0.82 1.00 2.87 0.250.37 0.57 0.15 0.00 0.04 0.27 0.01 Sample 12 1.03 0.69 1.00 2.84 0.930.63 0.63 0.14 0.00 0.05 0.04 0.00 Sample 13 1.05 0.96 1.00 3.22 0.660.47 0.61 0.14 0.00 0.03 0.09 0.03 Sample 14 1.06 1.04 1.00 3.13 0.340.77 0.77 0.15 0.00 0.05 0.34 0.04 Sample 15 1.07 0.84 1.00 3.42 0.580.60 0.23 0.18 0.00 0.08 0.27 0.00 Sample 16 1.08 1.08 1.00 3.10 0.660.59 0.04 0.16 0.00 0.03 0.16 0.02 Sample 17 1.13 1.29 1.00 3.70 0.830.87 0.00 0.19 0.00 0.04 0.20 0.02

TABLE 6 Relative value under condition where concentration of Co is 1Li/Co Li Co O C F Mg Ti Si Ca Na S Sample 21 1.00 1.08 1.00 3.23 0.470.45 0.49 0.20 0.00 0.06 0.35 0.01 Sample 22 1.03 0.73 1.00 3.05 1.060.70 0.59 0.17 0.00 0.03 0.09 0.00 Sample 23 1.05 1.19 1.00 3.40 0.730.48 0.58 0.23 0.00 0.05 0.10 0.04 Sample 24 1.06 1.10 1.00 3.44 0.330.92 0.91 0.25 0.00 0.05 0.30 0.04 Sample 25 1.07 1.05 1.00 4.26 0.740.20 0.25 0.29 0.00 0.07 0.30 0.02 Sample 26 1.08 1.12 1.00 3.32 0.740.52 0.14 0.21 0.00 0.05 0.15 0.00 Sample 27 1.13 1.49 1.00 3.80 0.600.41 0.00 0.12 0.00 0.00 0.16 0.02 Sample 28 1.05 0.99 1.00 3.11 0.720.86 0.56 0.18 0.00 0.05 0.12 0.02

TABLE 7 Relative value under condition where concentration of Co is 1Li/Co Li Co O C F Mg Ca Na Sample 31 1.00 0.51 1.00 2.45 0.69 0.08 0.270.03 0.08 Sample 32 1.01 0.67 1.00 2.65 0.77 0.08 0.28 0.03 0.08 Sample33 1.02 0.53 1.00 2.51 0.66 0.09 0.27 0.02 0.06 Sample 34 1.03 0.79 1.002.93 0.92 0.09 0.35 0.04 0.14 Sample 35 1.04 0.65 1.00 2.33 0.48 0.110.32 0.03 0.11 Sample 36 1.04 0.69 1.00 2.73 0.56 0.11 0.38 0.05 0.16Sample 37 1.05 0.67 1.00 3.04 0.64 0.09 0.35 0.04 0.21 Sample 38 1.060.83 1.00 2.65 1.03 0.29 0.10 0.05 0.11 Sample 39 1.08 0.80 1.00 2.791.04 0.26 0.03 0.08 0.10 Sample 40 1.13 0.77 1.00 2.72 0.22 0.99 0.000.01 0.26

FIGS. 46A and 46B are graphs in which the relative value of magnesiumand the relative value of titanium are extracted from the analysisresults of Tables 5 to 7. FIG. 46A is a graph showing the ratio of Li toCo and the relative value of magnesium. FIG. 46B is a graph showing theratio of Li to Co and the relative value of titanium.

From the analysis results of Samples 31 to 40 in FIG. 46A, it is foundthat, in the case where a coating layer containing titanium is notincluded, the concentration of magnesium is high in samples where theratio of Li to Co is greater than or equal to 1.00 and less than orequal to 1.05. This is probably because magnesium contained in thestarting materials is segregated in a range where the elementconcentration can be detected by XPS by heating. In contrast, in sampleswhere the ratio of Li to Co is greater than or equal to 1.06, theconcentration of magnesium is low; thus, it is probable that thesegregation of magnesium does not easily occur when the amount oflithium is too large.

From the analysis results of Samples 11 to 16 and Samples 21 to 26 inFIG. 46A, it is found that the concentration of magnesium in a rangewhere the element concentration can be detected by XPS is higher in thecase where a region containing titanium is included in a superficialportion than in the case where the region containing titanium is notincluded.

Moreover, in the case where the ratio of Li to Co is 1.06, in sampleswhere the region containing titanium is not included, the concentrationof magnesium in a range where the element concentration can be detectedby XPS is low; in contrast, in samples where the region containingtitanium is included, the concentration of magnesium in a range wherethe element concentration can be detected by XPS is high. That is, whenthe region containing titanium is formed in the superficial portion,magnesium is sufficiently segregated even in the case where the ratio ofLi to Co is high.

Note that even if the region containing titanium is included, theconcentration of magnesium is lower in the case where the ratio of Li toCo is 1.07 than in the case where the ratio of Li to Co is 1.06.Furthermore, it is probable that, in the case where the ratio of Li toCo is greater than or equal to 1.08, the segregation of magnesium doesnot easily occur even if the region containing titanium is included.

[Evaluation of Cycle Characteristics] <<Energy Density Retention Rate>>

Next, cycle characteristics were evaluated in a manner similar to thatof Example 1 using positive electrode active materials of Samples 11 to14, Sample 16, Samples 21 to 24, and Sample 26.

The shape of the secondary battery, the materials and the mixture ratiosof the positive electrode active material, the conductive additive, andthe binder in the positive electrode, the counter electrode, theelectrolyte solution, the exterior body, the conditions of the cyclecharacteristics test, and the like are the same as those in Example 1.

FIG. 47A is a graph showing the energy density retention rates and thenumber of charge and discharge cycles at the time of charging at 4.6 Vof secondary batteries using the positive electrode active materials ofSamples 11 to 14 and Sample 16 which were formed so that the amount ofTTIP per weight of the positive electrode active material was 0.01 ml/g.FIG. 47B is a graph showing the energy density retention rates and thenumber of charge and discharge cycles at the time of charging at 4.6 Vof secondary batteries using the positive electrode active materials ofSamples 21 to 24 and Sample 26 which were formed so that the amount ofTTIP per weight of the positive electrode active material was 0.02 ml/g.

As shown in FIG. 47A, in the case where TTIP is 0.01 ml/g, Samples 11 to14, that is, the positive electrode active materials in which the ratioof Li to Co is greater than or equal to 1.00 and less than or equal to1.06 have excellent cycle characteristics. In particular, Samples 11 and12, that is, the positive electrode active materials in which the ratioof Li to Co is greater than or equal to 1.00 and less than or equal to1.03 have extremely excellent cycle characteristics. In contrast, inSample 16 in which the ratio of Li to Co is 1.08, the energy densityretention rate decreases at a relatively early stage.

As shown in FIG. 47B, in the case where TTIP is 0.02 ml/g, Samples 21 to24, that is, the positive electrode active materials in which the ratioof Li to Co is greater than or equal to 1.00 and less than or equal to1.06 have excellent cycle characteristics. In particular, Samples 23 and24, that is, the positive electrode active materials in which the ratioof Li to Co is greater than or equal to 1.05 and less than or equal to1.06 have extremely excellent cycle characteristics.

FIG. 48 is a graph showing comparison between Sample 11 having the mostexcellent cycle characteristics in Samples 11 to 15 and Sample 23 havingthe most excellent cycle characteristics in Samples 21 to 25.

As shown in FIG. 48, both Sample 11 and Sample 23 have excellent cyclecharacteristics; however, Sample 23 in which TTIP is 0.02 ml/g has moreexcellent cycle characteristics.

<<Discharge Capacity Retention Rate>>

Next, FIG. 49 shows evaluation results of a discharge capacity retentionrate, which is one of the cycle characteristics of each of Samples 21 to26 and Sample 28.

The shape of the secondary battery, the material and the mixture ratioof the positive electrode active material, the conductive additive, andthe binder in the positive electrode, the counter electrode, theelectrolyte solution, the exterior body, the conditions of the cyclecharacteristics test, and the like of Samples 21 to 26 are the same asthose in Example 1.

The secondary battery using Sample 28 was formed in a manner similar tothose of the secondary batteries using Samples 21 to 26 except that PVDFwas used as a binder and the positive electrode active material (LCO),AB, and PVDF were mixed such that the weight ratio of LCO to AB and PVDFwas 95:3:2, and evaluated.

As shown in FIG. 49, Samples 21 to 24 and Sample 28 have excellent cyclecharacteristics. In particular, Sample 28 has extremely excellent cyclecharacteristics. In Sample 28, the discharge capacity retention rateafter 50 cycles was higher than or equal to 85%.

In contrast, in Sample 25 and Sample 26 in which the ratios of Li to Coare 1.07 and 1.08, respectively, the discharge capacity retention ratesdecrease at a relatively early stage.

From the above results, it is found that in the case where TTIP perweight of the positive electrode active material is 0.02 ml/g, the ratioof Li to Co preferably has a range of greater than or equal to 1.00 andless than 1.07. Moreover, it is found that a sample in which the ratioof Li to Co has a range of greater than or equal to 1.05 and less thanor equal to 1.06 has extremely excellent cycle characteristics.

FIGS. 50A to 50C show charge and discharge curves of the secondarybatteries using Sample 28, which has extremely excellent cyclecharacteristics in FIG. 49, Sample 24, and Sample 25, in which thedegradation occurs at a relatively early stage.

FIG. 50A, FIG. 50B, and FIG. 50C show charge and discharge curves of thesecondary batteries using Sample 28, Sample 24, and Sample 25,respectively. Each of the figures shows overlap of results of 50 cyclesof charge and discharge. As indicated by an arrow in each figure, thecharge and discharge capacity decreases from the first cycle to thefiftieth cycle.

As shown in FIGS. 50A and 50B, Sample 28 and Sample 24, which arepositive electrode active materials of one embodiment of the presentinvention, have high charge and discharge capacity and excellent chargeand discharge characteristics. In addition, it is found that a decreasein charge and discharge capacity of each of Sample 28 and Sample 24 inFIGS. 50A and 50B is significantly suppressed as compared with Sample 25in FIG. 50C.

Example 4

In this example, SEM observation results and SEM-EDX analysis results ofthe positive electrode active material of Sample 24 formed in Example 2are described.

Sample 24 was formed so that Li/Co was 1.06 and TTIP per weight of thepositive electrode active material was 0.02 ml/g. FIG. 51A shows a SEMimage of Sample 24. FIGS. 51B and 51C each show an enlarged image of apart in FIG. 51A.

As shown in FIGS. 51A to 51C, there are a large number of projectedregions in a superficial portion of the positive electrode activematerial.

Next, FIGS. 52A-1, 52A-2, 52B-1, 52B-2, 52C-1, and 52C-2 show analysisresults of the positive electrode active material of Sample 24 usingSEM-EDX. FIG. 52A-1 shows a SEM image of a superficial portion of thepositive electrode active material, FIG. 52A-2 shows a mapping image oftitanium, FIG. 52B-1 shows a mapping image of magnesium, FIG. 52B-2shows a mapping image of oxygen, FIG. 52C-1 shows a mapping image ofaluminum, and FIG. 52C-2 shows a mapping image of cobalt. Note that inEDX element mapping images in FIGS. 52A-2, 52B-1, 52B-2, 52C-1, and52C-2, a region where the number of elements is less than or equal to alower limit of the detection is indicated in black, and as the number ofelements is increased, the black region becomes white.

The same regions in FIGS. 52A-1, 52A-2, and 52B-1 are surrounded bydotted lines. When the regions surrounded by dotted lines are comparedwith each other, it is found that in the projected regions in thesuperficial portion of the positive electrode active material, titaniumand magnesium are distributed.

Thus, it is found that Sample 24 is a positive electrode active materialincluding the projected fourth region 104 containing titanium andmagnesium over the third region 103.

As shown in Example 2, Sample 24 is one of the samples which showextremely excellent cycle characteristics. Thus, it is found that, evenif the fourth region is provided in the superficial portion or when thefourth region is provided, the positive electrode active material withexcellent cycle characteristics can be obtained.

From the above results of Examples 1 to 3, it is found that when theregion containing titanium is formed in the superficial portion, thepositive electrode active material with excellent cycle characteristicscan be obtained. In addition, it is found that, when the ratio of Li toCo is increased to increase the particle diameter of the positiveelectrode active material, the cycle characteristics might be degraded;however, the region containing titanium is formed in the superficialportion, whereby the range of the ratio of Li to Co in which excellentcycle characteristics is obtained can be widened. Furthermore, it isfound that even when the fourth region containing titanium and magnesiumis provided in the superficial portion of the positive electrode activematerial, excellent cycle characteristics is obtained.

Example 5

In this example, an example of a method for producing a positiveelectrode active material coated with graphene oxide is shown, andobservation results of the positive electrode active material producedby the method with an electron microscope are described.

As shown in a process flow chart in FIG. 53, a process for forming acoating film on a positive electrode active material includes the stepsof: weighing graphene oxide (S11); mixing and stirring the grapheneoxide and pure water (S12); controlling pH (S13); adding an activematerial (S14); completing a suspension (S15); spraying the suspensionusing a spray dry apparatus (S16); and collecting particles in acontainer (S17).

Note that in (S12), pure water is used as a dispersion medium; however,a dispersion medium is not particularly limited and ethanol or the likemay be used. In addition, in (S14), the active material is a positiveelectrode active material.

FIG. 54 is a schematic view of a spray dry apparatus 280. The spray dryapparatus 280 includes a chamber 281 and a nozzle 282. A suspension 284is supplied to the nozzle 282 through a tube 283. The suspension 284 issupplied from the nozzle 282 to the chamber 281 in the form of mist anddried in the chamber 281. The nozzle 282 may be heated with a heater285. Here, a region of the chamber 281 which is close to the nozzle 282,for example, a region surrounded by dashed-two dotted line in FIG. 54,is also heated with the heater 285.

In the case of using a suspension containing a positive electrode activematerial and graphene oxide as the suspension 284, powder of thepositive electrode active material coated with the graphene oxide iscollected in a collection container 286 through the chamber 281.

The air in the chamber 281 may be suctioned by an aspirator or the likethrough a path indicated by an arrow 288.

An example of conditions for forming the coating film is shown below.

First, a suspension was formed by dispersing graphene oxide into asolvent.

Although in pure water, graphene oxide is highly dispersible, pure watermight react with an active material added later, so that Li might bedissolved or an active material might be damaged to change the surfacestructure. Thus, the graphene oxide was dispersed into a solution suchthat the ratio between ethanol and pure water was 4:6.

Stirring to disperse the graphene oxide into a solution was performedunder the following conditions: a stirrer and an ultrasonic wavegenerator were used; a rotation rate was 750 rpm; and irradiation timewith ultrasonic waves was 2 minutes.

Then, a LiOH aqueous solution was dropped to adjust pH to be pH7 (25°C.).

The positive electrode active materials (in this example, lithium cobaltoxide particles produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.(product name: C-20F)) were added, and stirring was performed using astirrer and an ultrasonic wave generator under the following conditions:a rotation rate was 750 rpm; and irradiation time with ultrasonic waveswas 1 minute. Through the above process, the suspension was prepared.The above lithium cobalt oxide particles produced by NIPPON CHEMICALINDUSTRIAL CO., LTD. (product name: C-20F) contain at least fluorine,magnesium, calcium, sodium, silicon, sulfur, and phosphorus, and eachhave a diameter of approximately 20 μm.

Next, the suspension was sprayed uniformly with a spray nozzle (having anozzle diameter of 20 μm) of the spray dry apparatus to obtain powder.The inlet temperature was 160° C. and the outlet temperature was 40° C.as the hot-air temperature of the spray dry apparatus, and the N₂ gasflow rate was 10 L/min.

FIG. 55 shows a cross-sectional TEM image of the obtained powder. Inaddition, FIG. 56 shows a SEM image of the obtained powder. When apositive electrode active material which was the same as the sprayedpositive electrode active material (C-20F, produced by NIPPON CHEMICALINDUSTRIAL CO., LTD.) was mixed with the graphene oxide with a planetarycentrifugal mixer as a comparative example, coating was insufficient.FIG. 57 shows a SEM image of the comparative example.

It is found that, as compared with FIG. 57, the coating film isuniformly formed on the surface of the powder in FIG. 56.

FIGS. 58A and 58B illustrate a cross-sectional structure example of anactive material layer 200 which is coated with graphene oxide with aspray dry apparatus and includes a graphene compound as a conductiveadditive.

FIG. 58A is a longitudinal sectional view of the active material layer200. The active material layer 200 includes positive electrode activematerial particles 100 coated with graphene oxide, a graphene compound201 as a conductive additive, and a binder (not illustrated). Here,graphene or multilayer graphene may be used as the graphene compound201, for example. The graphene compound 201 preferably has a sheet-likeshape. The graphene compound 201 may have a sheet-like shape formed of aplurality of sheets of multilayer graphene and/or a plurality of sheetsof graphene that partly overlap with each other.

In the longitudinal section of the active material layer 200, asillustrated in FIG. 58B, the positive electrode active material 100coated with a coating film 105 formed of graphene oxide is in contactwith the graphene compound 201. A plurality of graphene compounds 201are formed in such a way as to be partly in contact with the positiveelectrode active material 100 coated with the coating film 105 andadhere to the coating film 105 of the adjacent positive electrode activematerial 100, so that the graphene compounds 201 are in contact witheach of the positive electrode active material 100.

The graphene compound 201 and the coating film 105 are formed usingcarbon-based materials; thus, an excellent conductive path can beformed.

The coating film 105 is effective in protecting the crystal structure ofthe positive electrode active material 100 so as not to be in contactwith the electrolyte solution and in forming the excellent conductivepath.

REFERENCE NUMERALS

11 a: positive electrode, 11 b: negative electrode, 12 a: lead, 12 b:lead, 14: separator, 15 a: bonding portion, 15 b: bonding portion, 17:fixing member, 50: secondary battery, 51: exterior body, 61: foldedportion, 62: seal portion, 63: seal portion, 71: crest line, 72: troughline, 73: space, 100: positive electrode active material, 101: firstregion, 101 p: crystal plane, 102: second region, 102 p: crystal plane,103: third region, 103 p: crystal plane, 104: fourth region, 105:coating film, 106: crack portion, 110: particle, 111: region, 112: layercontaining titanium, 114: cobalt oxide layer, 120: particle, 121:region, 122: layer containing titanium, 124: cobalt oxide layer, 125:layer containing lithium titanate, 200: active material layer, 201:graphene compound, 214: separator, 280: spray dry apparatus, 281:chamber, 282: nozzle, 283: tube, 284: suspension, 285: heater, 286:collection container, 288: arrow, 300: coin-type secondary battery, 301:positive electrode can, 302: negative electrode can, 303: gasket, 304:positive electrode, 305: positive electrode current collector, 306:positive electrode active material layer, 307: negative electrode, 308:negative electrode current collector, 309: negative electrode activematerial layer, 310: separator, 500: laminated secondary battery, 501:positive electrode current collector, 502: positive electrode activematerial layer, 503: positive electrode, 504: negative electrode currentcollector, 505: negative electrode active material layer, 506: negativeelectrode, 507: separator, 508: electrolyte solution, 509: exteriorbody, 510: positive electrode lead electrode, 511: negative electrodelead electrode, 600: cylindrical secondary battery, 601: positiveelectrode cap, 602: battery can, 603: positive electrode terminal, 604:positive electrode, 605: separator, 606: negative electrode, 607:negative electrode terminal, 608: insulating plate, 609: insulatingplate, 611: PTC element, 612: safety valve mechanism, 613: conductiveplate, 614: conductive plate, 615: module, 616: wiring, 617: temperaturecontrol device, 900: circuit board, 910: label, 911: terminal, 912:circuit, 913: secondary battery, 914: antenna, 915: antenna, 916: layer,917: layer, 918: antenna, 920: display device, 921: sensor, 922:terminal, 930: housing, 930 a: housing, 930 b: housing, 931: negativeelectrode, 932: positive electrode, 933: separator, 950: wound body,951: terminal, 952: terminal, 980: laminated secondary battery, 981:film, 982: film, 993: wound body, 994: negative electrode, 995: positiveelectrode, 996: separator, 997: lead electrode, 998: lead electrode,7100: portable display device, 7101: housing, 7102: display portion,7103: operation button, 7104: secondary battery, 7200: portableinformation terminal, 7201: housing, 7202: display portion, 7203: band,7204: buckle, 7205: operation button, 7206: input output terminal, 7207:icon, 7300: display device, 7304: display portion, 7400: mobile phone,7401: housing, 7402: display portion, 7403: operation button, 7404:external connection port, 7405: speaker, 7406: microphone, 7407:secondary battery, 7408: lead electrode, 7409: current collector, 7500:vaporizer, 7501: atomizer, 7502: cartridge, 7504: secondary battery,8000: display device, 8001: housing, 8002: display portion, 8003:speaker portion, 8004: secondary battery, 8021: ground-based chargingapparatus, 8022: cable, 8024: secondary battery, 8100: lighting device,8101: housing, 8102: light source, 8103: secondary battery, 8104:ceiling, 8105: wall, 8106: floor, 8107: window, 8200: indoor unit, 8201:housing, 8202: air outlet, 8203: secondary battery, 8204: outdoor unit,8300: electric refrigerator-freezer, 8301: housing, 8302: refrigeratordoor, 8303: freezer door, 8304: secondary battery, 8400: automobile,8401: headlight, 8406: electric motor, 8500: automobile, 8600: motorscooter, 8601: side mirror, 8602: secondary battery, 8603: indicator,8604: storage unit under seat, 9600: tablet terminal, 9625: power savingmode changing switch, 9626: display mode changing switch, 9627: powerswitch, 9628: operation switch, 9629: fastener, 9630: housing, 9630 a:housing, 9630 b: housing, 9631: display portion, 9631 a: displayportion, 9631 b: display portion, 9633: solar cell, 9634: charge anddischarge control circuit, 9635: power storage unit, 9636: DCDCconverter, 9637: converter, 9640: movable portion.

This application is based on Japanese Patent Application Serial No.2016-133997 filed with Japan Patent Office on Jul. 6, 2016, JapanesePatent Application Serial No. 2016-133143 filed with Japan Patent Officeon Jul. 5, 2016, Japanese Patent Application Serial No. 2017-002831filed with Japan Patent Office on Jan. 11, 2017, Japanese PatentApplication Serial No. 2017-030693 filed with Japan Patent Office onFeb. 22, 2017, Japanese Patent Application Serial No. 2017-084321 filedwith Japan Patent Office on Apr. 21, 2017, and Japanese PatentApplication Serial No. 2017-119272 filed with Japan Patent Office onJun. 19, 2017 the entire contents of which are hereby incorporated byreference.

1. A method for forming a positive electrode active material, the methodcomprising the steps of: preparing a particle comprising a compositeoxide, the composite oxide containing lithium, cobalt, magnesium andfluorine; coating the particle with a material containing titanium; andheating the particle coated with the material containing titanium sothat magnesium and fluorine contained in an inside of the particle aresegregated on a surface of the particle and that titanium contained inthe material containing titanium is diffused into the inside of theparticle.
 2. The method according to claim 1, wherein the particle iscoated with the material containing titanium by a sol-gel method.
 3. Themethod according to claim 1, wherein the particle comprising thecomposite oxide is prepared by a process comprising: mixing a source oflithium, a source of cobalt, a source of magnesium and a source offluorine to form a mixture; and heating the mixture at 800° C. or higherand 1100° C. or lower.
 4. A method for forming a positive electrodeactive material, the method comprising the steps of: preparing aparticle comprising a composite oxide, the composite oxide containinglithium, cobalt, magnesium and fluorine; coating the particle with amaterial containing titanium; and heating the particle coated with thematerial containing titanium to cause segregation of magnesium andfluorine and diffusion of titanium, resulting in that, in a lineanalysis of energy dispersive X-ray spectrometry, a peak of aconcentration of magnesium and a peak of a concentration of fluorine arepositioned closer to a surface of the positive electrode active materialthan a peak of a concentration of titanium.
 5. The method according toclaim 4, wherein the particle is coated with the material containingtitanium by a sol-gel method.
 6. The method according to claim 4,wherein the particle comprising the composite oxide is prepared by aprocess comprising: mixing a source of lithium, a source of cobalt, asource of magnesium and a source of fluorine to form a mixture; andheating the mixture at 800° C. or higher and 1100° C. or lower.
 7. Themethod according to claim 4, wherein a region in which a concentrationof titanium is equal to or higher than ½ of a peak of titanium is formedby the step of heating in a range from the surface of the positiveelectrode active material to a depth of 20 nm.
 8. The method accordingto claim 4, wherein the surface of the positive electrode activematerial is positioned at a measurement point at which a measurementvalue of oxygen in the line analysis of energy dispersive X-rayspectrometry is 0.5×Oave, where Oave is an average value of themeasurement value of oxygen in the line analysis of the energydispersive X-ray spectrometry in a region where the measurement value ofoxygen is stable.
 9. The method according to claim 4, wherein the peakof titanium is present in a region from a depth of 0.2 nm or more to adepth of 10 nm or less.
 10. A method for forming a positive electrodeactive material, the method comprising the steps of: preparing aparticle comprising a composite oxide, the composite oxide containinglithium, cobalt, magnesium and fluorine, the particle comprising a crackportion; coating the particle with a material containing titanium; andheating the particle coated with the material containing titanium sothat magnesium and fluorine contained in an inside of the particle aresegregated on a surface of the particle and in the crack portion andthat titanium contained in the material containing titanium is diffusedinto the inside of the particle.
 11. The method according to claim 10,wherein the particle is coated with the material containing titanium bya sol-gel method.
 12. The method according to claim 10, wherein theparticle comprising the composite oxide is prepared by a processcomprising: mixing a source of lithium, a source of cobalt, a source ofmagnesium and a source of fluorine to form a mixture; and heating themixture at 800° C. or higher and 1100° C. or lower.
 13. The methodaccording to claim 10, wherein titanium is segregated in the crackportion.
 14. A method for forming a positive electrode active material,the method comprising the steps of: preparing a particle comprising acomposite oxide, the composite oxide containing lithium, cobalt,magnesium and fluorine, the particle comprising a crack portion; coatingthe particle with a material containing titanium; and heating theparticle coated with the material containing titanium to causesegregation of magnesium and fluorine and diffusion of titanium,resulting in that, in a line analysis of energy dispersive X-rayspectrometry, a peak of a concentration of magnesium and a peak of aconcentration of fluorine are positioned closer to a surface of thepositive electrode active material than a peak of a concentration oftitanium, wherein magnesium is segregated at least in the crack portion.15. The method according to claim 14, wherein the particle is coatedwith the material containing titanium by a sol-gel method.
 16. Themethod according to claim 14, wherein the particle comprising thecomposite oxide is prepared by a process comprising: mixing a source oflithium, a source of cobalt, a source of magnesium and a source offluorine to form a mixture; and heating the mixture at 800° C. or higherand 1100° C. or lower.
 17. The method according to claim 14, wherein aregion in which a concentration of titanium is equal to or higher than ½of a peak of titanium is formed by the step of heating in a range fromthe surface of the positive electrode active material to a depth of 20nm.
 18. The method according to claim 14, wherein the surface of thepositive electrode active material is positioned at a measurement pointat which a measurement value of oxygen in the line analysis of energydispersive X-ray spectrometry is 0.5×Oave, where Oave is an averagevalue of the measurement value of oxygen in the line analysis of theenergy dispersive X-ray spectrometry in a region where the measurementvalue of oxygen is stable.
 19. The method according to claim 14, whereinthe peak of titanium is present in a region from a depth of 0.2 nm ormore to a depth of 10 nm or less.
 20. The method according to claim 14,wherein titanium is segregated in the crack portion.
 21. A method forforming a positive electrode active material, the method comprising thesteps of: preparing a particle comprising a composite oxide, thecomposite oxide containing lithium, cobalt, magnesium and fluorine;coating the particle with a material containing titanium; and heatingthe particle coated with the material containing titanium to causesegregation of magnesium and fluorine and diffusion of titanium, whereina relative value of a concentration of titanium is greater than or equalto 0.05 and less than or equal to 0.4 when a surface of the positiveelectrode active material is subjected to an XPS analysis and aconcentration of cobalt is defined as 1.